Pink Pigtail Inn has what is probably the most involved guild anniversary I've ever seen. We've seen quite a few anniversaries and events come through Guildwatch, but this one takes the cake: a huge competitive scavenger hunt, complete with out-of-game clues, banned class abilities (so teams could be balanced out), and even self-made quests involving the guild's lore. It's probably rare to find a group of officers that can be this committed to something normally considered "an RP event," but obviously it worked out, because the whole guild really enjoyed it.
The context Larisa puts this in, however, is even more interesting. According to the Daedalus Project (a series of surveys of MMO players -- we've mentioned their work before), the majority of players can't celebrate a guild's anniversary anyway, as they haven't been in their guilds for even a year yet. I've never considered it, but it's true: while we are very attached to our guildies when we do find a good guild, we aren't really attached to them for very long, relatively speaking. There are stories of guilds going on for decades, but even those guilds have players coming and going -- if your guild has the same group of people playing together for a few years, you're probably in a smaller group than you think. PPI's example is a great one for any guilds who have been around long enough to celebrate it.