There are a variety of websites that have lists of improv games. Some are comprehensive lists that are compiled with the help of many contributors. Some are the list of games a particular troupe use for their shows. As of today, Sept. 6, 2006, this list is accurate. I will try and update this post periodically with any changes that I find. Improv games often are known by several names depending on the group referring to it. Also, the game may have several variant methods of playing it.
Unfortunately, there isn't a single site that I go to for all games. Some are more comprehensive than others (listed first, below) , and I still review small sites to find any new or unique games. New games and variations are being developed all of the time.
Improv Encyclopedia has the most comprehensive list, and provides the games either alphabetically or by categories. They also try and list the games under all known names and variants. I use this site most often.
Chicago Improv Network Wiki is a new site to me. They have three lists, Exercises, Longforms, and Shortforms. I really like the layout, but there is a problem with a lot of incomplete definitions. In several cases, a page for a game is exists but without content. I plan on spending some time exploring this site.
The Improv Wiki has a small set of games, and doesn't appear to be updated very often. However, it's worth checking in on this one from time to time.
Learn Improv has a smaller list, but the descriptions are pretty good and their list is also categorized. Access their lists using the Structures box on the side.
The Living Playbook hasn't been updated for awhile (the current list is dated from 2001), but it is a long list of long and short form games along with a glossary.
Long Form Stuff Shaun Has ... is a list of long form structures that Chicago's Shaun Himmerick has collected from a variety of source. The descriptions are pretty good although brief.
Idiotica has a smaller list, and the descriptions aren't detailed, but they do try to rate the characteristics of each game. Each game is given a score between 0 and 5 for Character, Plot, Timing, Mime, and Group Mind.
Fuzzy's Game List is an older list that hasn't been updated for awhile, but is still out there and worth looking over.
Mark's Guide to Whose Line Is It Anyway, The Games is a fun site where a lot of the games seen on the Who's Line Is It Anyway TV show are listed. The descriptions are limited, but they include a sample of the dialog. It's a fun list.
The Spolin Games list games promoted by The Spolin Center. The group promotes the work of Viola Spolin, one of the founders of modern improv. The descriptions are way too short, but they do give you a sense of the games.
FNI Games is a site listing the games played by the FNI (Friday Night Improv) in Pittsburgh. A short list, but I sometimes like seeing this kind of focused list.
Improv Games for Rehearsal and Performance is an interesting site that has a short description of a large group of games. I'm not certain who put this list together, but it's a nice list.
Improv Games for Kids is a nice little list of improv games that are appropriate for kids.