To be honest, probably my favourite site - a veritable Aladdin's cave of goodies - and think maybe every player should be required to read the etiquette section.
Play motif for a quick bit of fun. Well recommended.
Mike Cranes site and probably the home of BG in this country. Great starter tutorials.
Has major competitions, such World championship, nearby in Daventry.
Not the prettiest of sites, just me - I don't like frames and its not W3C compliant - being a nerd. Well worth visiting though.
Turn based site. Free option. Tons of games to choose from. I play here too, but becoming a bit disillusioned - slow servers, too much distraction from expansion of site to offer more and more games. Still - give it a go - good clubs and some good players ( even though I'm 14th highest !! )