- Namecheap (my favourite, all my domains are here, excellent customer service - the Zappos of domains - instant support, live chat 24/7, friendly/educated staff, and just a real pleasure to use)
- Domain Monster - UK/European based domains. Good services, good response times
- Europe Registry / Instra - used this commercially, more expensive, not the greatest of control panels, but more than likely you will be appointed an account manager (hence good for businesses) and pretty much have the largest database of domain extensions to register against. If you are interested in moving to them, I can also put you in touch with a very friendly chap who can help you make the move.
I hope this information helps - reply me up on @o6uoq if you'd like to chat about anything else :)