Didn't have much time to prepare this year, but one of our mods got the idea to change the name "Beatles" into a censored word, and our censor character was a heart symbol.

(We're a music forum.)
In the past, we changed everyone's avatar over so they had the same one (a simple template edit to point to a static image). Nobody knew who anyone else was! (Avatars really become the visual identity of a forum member over time!) Another year, we added a Nauga to some busier members' avatars (
http://www.nauga.com/), as the Nauga has been a running joke on our forum for years.
One of the most clever ones we had was when someone posted that our forum was being bought out by Clear Channel (which is a big radio conglomerate). IOW, we were selling out.
I think the one that got us the most attention, including notoriety on other forums and sites, was when someone convincingly posted news that there were some new Beatles remasters being released. I mean, it was so notorious that it even got the attention of the studios, record labels, etc. It was a good prank, but it was also a little too serious. Fortunately everyone involved had a good chuckle about it, but considering the consequences we could have had, we haven't allowed anything that drastic since then.