I always use http://www.providerdomain.de .
You're paying only 60 % of the price United Domains charges and you have good service (at least I only had it).
Another registrar which somebody recommended me is http://www.nwx.biz
I haven't had any experiences with them yet, but it looks a bit strange because their server seems to be down right now.
Hope I could help...
PS: Unfortunately both websites are only available in German