I think
this (click me!) may be what you're looking for. My old guild used this for awhile and it worked out pretty well (while people actually used it.) It basically allows everyone in your guild to share a calendar where you can set up events (raids, guild meetings, etc.) and people can either just view than, sign up for them, or say they won't be able to attend.
Aranan: The version on WoW Interface is an older one from February. The link above from Curse has the latest release.