I use freeSide, - at the moment it integrates poorly with Xen, but
I'm working on that. freeSide is open source.
If other people wanted to help integrate freeSide with Xen, I'd be happy
to work with others; adding good Xen integration to freeSide would help
many of us. Everything I've done so far has been posted on the
freeside-devel mailing list.
But then, if you are looking for something that is already done, well,
that's not it. (I mean, freeSide is done, but it's Xen integration isn't.)
right now I use freeSide for billing, and a collection of shell scripts
that let a user get console to his or her DomU. I do the actual provisioning