The BGGDL (Board Game Geek Dominion League) was playing Dominion competitively on http://dominion.isotropic.org.
This league is currently inactive; the last round ended in June 2011.
Latest rules are always available on the rules wiki. Note there are significant changes for round 19: Rules Wiki
Results from previous rounds can be found here: BGGDL Results
For anyone still interested in Tournaments and Leagues, they pop up every once in a while here: Dominion Strategy Tournaments