A (non-exhaustive) list of resources that are useful for learning about, and getting better at, quizbowl.
General
NAQT's website, which contains a lot of information such as an introduction to quizbowl and NAQT's official ruleset.
ACF's website, which contains information such as ACF's competitions (Fall, Winter, Regionals, and Nationals) as well as ACF's official ruleset which is used for most tournaments not run on NAQT questions.
The quizbowl forums, the central location for tournament announcements, discussion threads, and many more topics not necessarily related to quizbowl.
Studying
The packet archive, which has a very large number of packets for studying and practicing. Most of the packets we read in practice are from the collegiate packet archive, but harder high school sets (those marked 3, 4, and 5) are in many cases comparable in difficulty to easy- and medium-difficulty college quizbowl sets.
aseemsDB and QB Reader, two searchable databases of quizbowl questions. QB Reader allows for filtering by question difficulty and question category/subcategory, but aseemsDB has a more comprehensive collection of questions.
NAQT's You Gotta Know, a monthly series of articles that are good introductions to various topics from across the quizbowl canon.
Wikipedia, obviously.
Other
The college quizbowl calendar created by Ophir Lifshitz, which contains information on announced question sets and tournaments for current and past seasons, along with information on college question difficulty.
Harry White's tournament search website, which is a convenient way to search for a school or player's past results.
The quizbowl Discord, where a lot of quizbowlers old and young socialize.