Now THIS is cool.
This site is a ridiculously useful resource for anyone who's a big fan of webcomics: The Webcomic Crossover and Cameo Archive. Wanna know any time someone's taken a shot at Penny Arcade? Now you can (sorry I linked out of the frameset there. You'll find your way back).
Not only does this site have an incredibly huge archive of crossovers, it's got a feature where you can report your own. So anytime a favorite comic of yours makes a clever reference that you think everyone else on the web missed, you can add it up there and revel in your own deductive ingenuity.
It's actually surprisingly fun to look up some of your favorite webcomics off the list and take a look at how other webcomickers have spoofed them and slipped them into their stories. Sure it's got an archive of the full-blown crossovers as well (such as Melonpool/It's Walky, and Goats/Bobbins), but if you're a follower of a strip which does a major crossover, you're obviously going to know that crossover already. It's a lot more fun to see all the random, off-the-cuff references your favorite comic has garnered over the years in the cameo section.
So check it out, add your own contributions, and enjoy!