I think it’s pretty much de rigeur now for MMO guilds to have their own websites, even if it is just a free bboard so that members can communicate when they aren’t in the game. Semi-professional guilds like Ensidia go the whole nine yards with forums, player blogs, articles and guides, advertising, and I’m quite surprised they don’t sell branded T-shirts and mugs too.
There are also specialist services like wowstead.com and guildportal.com which present guilds with the ability to easily set up complex and game-specific functionality (like rosters, contents of the guild bank, raid calendars, armoury access, etc). And I think EQ2 offers a similar service to guilds directly, although not for free, which I always thought was a fantastic idea.
For me personally, I’m happy with a basic bulletin board. Although calendars where you can sign up for guild events have also worked well for us.
What do you look for in a good guild website?