ESPN's Free Agent Tracker
You can't deny that ESPN's Free Agent Tracker is always a handy reference. The new one is out; bookmark it.
Though to nitpick I think they should include Japanese guys like Matsuzaka, Igawa, and Iwamura even though they're not technically free agents. Last year Steve Phillips left Kenji Johjima off his initial Top 50 Free Agents list and it was not cool.