There are at least four different lists of improv games being maintained on the Web.
The most in-depth (complete with playing hints, alternate names, etc) is Hugh's List of Improv Handles, maintained by Hugh McLeod (firstname.lastname@example.org).
The most extensive (i.e. the most entries) is The Living Playbook. While not as in-depth as Hugh's List, it's probably the most comprehensive reference.
There's also another list maintained by Fuzzy Gerdes (email@example.com). It's a work-in-progress, but it has some games that aren't on the other two lists.
For completeness, I would use all three. A good strategy is to check out The Living Playbook to pick a game, then examine Hugh's list for a more detailed description and playing hints. If you can't find a particular game on either list, check out the one maintained by Fuzzy Gerdes.