I’m fairly certain that Casino On Net are the biggest pure online casino. In the UK, you see their advertising in men’s magazines, on the tube (subway), they sponsor a major Premiership football club and various snooker and other sports events. Their brand presence over this side of the pond is pretty immense.
I don’t think any other online casino really compares on that level. It’s mainly the poker rooms that are getting their name out over here – Party Poker (who’ve just sponsored a poker tournament on terrestrial TV) in particular.