Is your Address Book full of web-savvy friends who know what a Gravatar (or a Pavatar) is? Avatars is a freeware plugin for Address Book that searches for, displays, and adds your contacts' avatars to their cards. It installs with a package installer as a SIMBL plugin, and it looks to me like SIMBL is in the package, too, just in case you need it.
It's simple, useful and has the right amount of eye candy to be visually interesting without being intrusive. Now I just need more friends with avatars.