There's several solid leagues still around that are dieing to find new team owners. Here's a few I'm in:
Around the Horn (ATH) -
http://ath.ootpleagues.com/index.php. I actually run this league now and it's a solid league, some good owners, and we have 3 teams open and possibly a 4th (Cubs) as the GM has went awol.
Canadian Baseball League (CBL) -
http://www.cbl-online.info/home.php
This might be the most active league I'm in. It's a tough league with very good GMs, very active GMs, and a heck of a challenge as it has a low salary cap which creates movement in the league and requires you to really budget your team.