In the UKt
Overture
Espotting.com (owned by findwhat)
Mirago.co.uk
The best conversions in my experience are Overture but the prices are very high, and I tend to target words after the 3/4
Mirago bid prices are the lowest but you have to work harder for conversions
but with the low prices it gives you flexibility to try stuff out without blowing all your money