The classicist’s killer app is the app crit.
Horrible puns aside: the apparatus criticus (app crit) is that big block of text at the bottom of any OCT or Teuber edition, and it shows the variants and conjectures for uncertain words in ancient texts. (As a pedagogical point, I find it valuable to show students a good scholarly Bible with an enormous app crit documenting the conflicts in manuscripts and textual traditions.) These guides to textual problems, however, are dense and loaded with inscrutable abbreviations, and even for advanced graduate students, they can be daunting.
At work, I recently discovered a helpful guide to the symbols and abbreviations one finds in an app crit, compiled by Karl Maurer at the University of Dallas. Many thanks to him for this helpful document, which I’ll add to this site’s list of classicist resources.