A few. If you have a Mac and don't mind buying the Mac version as well, Bento is a great little database. You need the Mac version because there is no other way to sync or export the data off the iPad. At least not yet. I keep hoping they'll decide it's worth making Bento a stand alone app on the iPad.
Otherwise you might want to look at HanDBase. I haven't used it, but it seems to be the closest thing to a full stand alone database on the iPad.