"But avoid foolish questions, and genealogies [...] for they are unprofitable and vain." (Titus 3:9)
The internet is full of amateur genealogy sites attempting to cover vast patches of history, particularly medieval and/or peerage history. Many of these sites are terrible. Some are quite good. The best personal all-my-ancestors site we're aware of is that of Janet Wolfe, a frequent and respected contributor to various scholarly genealogical periodicals; her site is a model of "showing your work", frequently quoting at length from stupendous amounts of primary-source information. Three more good all-my-ancestors sites are Powys-Lybbe Forbears by Tim Powys-Lybbe; Nancy López Genealogy by Nancy López, and the site of Hal Bradley, all of them well-sourced. Another very useful site is Leo's Genealogics by the late Leo van de Pas, a Dutch emigrant to Australia who collected genealogical data from reputable sources and posted it to to a giant online database for over two decades; by the time he died in 2016, his site had information on over 700,000 people. Genealogics is particularly useful for those of us who, like most of the English-speaking world, don't have easy access to a copy of the multi-volume Europäische Stammtafeln. Another important site, although not really "amateur," is The Henry Project, by historian Stewart Baldwin with assistance from Peter Stewart and Todd A. Farmerie, which is an attempt to set forth the direct ancestry of Henry II, King of England 1154-1189, based entirely on primary sources, or as close to those as history affords.
The Complete Peerage of England, Scotland, Ireland, Great Britain and the United Kingdom, Extant, Extinct or Dormant, frequently abbreviated "CP," is the standard modern reference work on the titled aristocracy of the British Isles, and therefore a central resource for anyone investigating their traceable medieval ancestry, since with only a few exceptions the only genealogical knowledge we have for the English-speaking world prior to about 1550 concerns titled or gentry persons. The Complete Peerage's first edition was edited by George Edward Cokayne, Clarenceux King of Arms, and published in eight volumes 1887-1898. The second edition, much revised, edited successively by Vicary Gibbs, H. A. Doubleday, Duncan Warrand, Lord Howard de Walden, Geoffrey H. White and R. S. Lea, was published in fourteen volumes 1910-1959 (Volumes 1-13, but volume 12 was two books, 12A and 12B). A Volume 14 consisting of "addenda & corrigenda", edited by Peter W. Hammond, appeared in 1998. The Wikipedia article about The Complete Peerage is here. The two things to keep in mind about CP, as it's generally abbreviated, are that (1) it came into being as a reaction to the shoddy genealogy being flogged in the 19th century by entrepreneurs like Burke and worse, and (2) for all its flaws, it is so much the standard that new discoveries in medieval and peerage genealogy are often published under the rubric of "corrections and additions to The Complete Peerage". As of 11 March 2017, searchable PDFs of all fourteen volumes are downloadable from the "Books" sections of the LDS site familysearch.org.
We do cite various heraldic visitation records, as published (sometimes well, often badly) in the 19th and 20th centuries. We do so fully aware that visitation genealogies can be problematic. Chris Phillips has an excellent explainer here. By and large, if we're citing only a visitation in support of an asserted fact, you should take it with a large grain of salt, because we certainly do. Even more skepticism should be applied when we cite any of the extruded genealogical products manufactured since 1826 by John Burke and his corporate descendants. We think that Burke's can be considered provisionally reliable for gentry families in the 17th to 20th centuries, at least as a finding aid and pointer to further research, but we avoid citing any Burke's product in connection with, for instance, events in the Middle Ages.
John A. Freestone's 1999 The Life and Times of Alonzo Hamilton Packer and His Wife Lydia Ann Parker (1999) and Donna Smith Packer's On Footings From the Past: The Packers in England (1988) are both useful, but both of them contain the kind of historical errors that call the whole work into question. The Life and Times, a self-published all-my-ancestors volume, is worse; for example, it blithely claims that the family of Anne Coates (1699-1785) "was of royal descent from King Charles I", which is obviously impossible. On Footings from the Past displays a greater fastidiousness, but just like the Life and Times, it reproduces an antique photograph of a bearded man in a large top hat, identifying him as Moses Packer (1764-1830). As we note on our page for Moses Packer, the earliest known photographs of human beings were made in 1839.
Both books detail an ancestry for Teresa's Packer forebears reaching back to architect and Royal Society member Philip Packer (1618-1686) of Groombridge Place, Kent, supposedly the father of the Philip Packer who emigrated to North America in 1682 or 1683 and settled in New Jersey. The Royal Society member Philip Packer certainly existed, as did the immigrant, but we'll list the former as the father of the latter when we can find some sources that are neither Packer descendants nor promoters of tourism to modern-day Groombridge Place. Meanwhile, we use both of these books as sources for the immigrant Philip Packer and his descendants, but with caution.
Col. Charles Wickliffe Throckmorton's 1930 A Genealogical and Historical Account of the Throckmorton Family in England and the United States, available for searching or browsing on Ancestry.com, is rich with useful information. The eminent genealogist G. Andrews Moriarty, himself a Throckmorton descendant, exchanged research with Charles Wickliffe Throckmorton and commended his Throckmorton study as a good source on the early generations of the line. But Throckmorton's Account is also notorious for having promulgated, in the chaper entitled "Besford Excursus", the belief that Alexander Besford, a Worcestershire knight of the shire who died about 1400 and who was an ancestor to (among other early New England immigrants) Alice Freeman of Connecticut and John Throckmorton of Rhode Island, was son to a Joan de Harley who was herself daughter of Joan Corbet, dau. of Sir Robert Corbet, and thus descended from Louis IV of France, Heinrich "the Fowler" of Germany, Llywelyn ap Iorwerth, the dukes of Normandy, and various other medieval eminences.
This false connection, which Throckmorton presented with a farrago of plausible-seeming contemporary documentation, made its way into innumerable otherwise-decent reference works including several editions of Ancestral Roots (though not the latest) and Henry J. Young's The Blackmans of Knight's Creek. (Although it's notable that Moriarty himself never accepted it, finding it dubious on simple chronological grounds.) It was finally and thoroughly disproved by John G. Hunt and Henry J. Young in their article "Ravens or Pelicans: Who was Joan de Harley?" (The Genealogist, n.s., 1:27, Spring 1980). Aside from showing beyond a shadow of a doubt that the Joan de Harley who granted Alexander de Besford certain rights in a property called Harley was a completely different individual from the much more highborn Joan de Harley who was the daughter of Joan Corbet, the authors also make several general observations about Throckmorton's methods which are applicable to a great deal of other bad genealogy, online and otherwise:
"In the scholarly articles of [G. Andrews] Moriarty the pedigree emerges inevitably from the original documents consulted; by contrast, in the Colonel's 'Besford Excursus', the Colonel uses his documents for verisimilitude and ornamentation, almost as a smoke screen, relying ultimately on his hunch. [...] Even his citations are lifted, not always accurately, without acknowledgement of the immediate source. Also, he confuses the regnal chronology in several places; twice he equates a date in 41 Edward III with 1359 instead of 1367, on another occasion he misdates a record of 34 Edward I as 9 Edward III, and he dates the reign of Abbot Gervaise of Pershore exactly 200 years too late."Nine years later, again in The Genealogist, Paul C. Reed came along and bounced the rubble. ("Another Look at Joan de Harley: Will Her Real Descendants Please Rise?" 10:1, Spring 1989.)
All that said, Throckmorton's book is full of useful information; the mass of documentation he presents may have been there for (as Hunt and Young say) "verisimilitude and ornamentation," but it is in fact present. When we cite Throckmorton's book, we're referring to the primary-source evidence presented there, not to the sometimes-spurious genealogical conclusions that C. Wickliffe Throckmorton imposed upon it.