Aria are great.
OverclockersUK have a huge selection - but their postage charges are a little expensive.
Scan also have a great choice, but their RMAs can sometimes be very poor, so I wouldn't recommend buying a motherboard from there, or anything else that has a high chance of going wrong, or anything that you could get elsewhere.
eBuyer are good - but they don't have the best selection of non-mainstream parts anymore.
CCL are good too. Don't have a huge selection, but they're still a good company.