I don't know of any sites that track that sort of information systematically, but maybe I can help you...
Dave Martinez is a free agent; whether he'll attract much interest at his age and after missing so much time due to injury remains in doubt. Too early to tell.
The chances of Tim Crabtree resurfacing seem pretty slim. His four games at Vero Beach this year don't provide much reason for optimism, unfortunately. I hope he can somehow make it back, but it seems the odds are against him, at least for now.
Powell seems to have found his groove in Japan - he pitched some great games for the Kintetsu Buffaloes this year. I think it's a bit early to speculate about when or if he'll return to the US; my guess is that he'll remain in Japan for the foreseeable future.
Loiselle has dropped off my radar screen after this year's spring training, when he was with the Cardinals as a non-roster invitee. I haven't heard of any clubs offering him another shot, but you never know.
Hope this helps!