You will also likely want to do two things to help the tags from spinning out of control: auto-complete and aliasing. The tags used by stackoverflow would be an outstanding point of reference, as they do both of those things, but in addition their tags would be (for all intents) a great list to get things started off. I haven't checked to see if their tag dataset is open, but I also wouldn't imagine it would be terribly hard to get the list, even if they aren't open. I would just avoid taking the descriptions that accompany them, unless that's offered under a liberal license.