Smarkets is the main one that is worth a look, they have smaller markets than Betfair (Ffos Las first race is 20k pool on their site compared to 32k on Betfair). I find their site easy to use and the market graphs and data are interesting.
They have fewer markets, and because of the smaller amounts in the pools it can be less competitive. I see Betfair as being the best one still, I have put my account down to the original terms and conditions (2%) rather than having their "offers".
If using an exchange I would always look for the smallest commission and biggest pools.
