Our discussion will be led, of course, by the extravagantly useful MLB Trade Rumors (sic). They've got a special feature up right now entitled "The 2010 Trade Market", although that just lists players who are available, not really a useful page for identifying potential trade partners. Here's the Blue Jays-specific feed.
With Alex Gonzalez gone, it looks like the main pieces the Jays will have to offer are Downs, Frasor, Gregg, Buck and Overbay. Bautista and Marcum have also been brought up in rumours at MLBTR, but are less likely to be dealt.
These teams are reasonably in a race and could be buying:
AL: Yankees, Red Sox, Rays, White Sox, Tigers, Twins, Rangers, Angels
NL: Braves, Mets, Phillies, Cardinals, Reds, Padres, Giants, Rockies, Dodgers
Okay, the next step is to identify which teams need which piece. It's late, so I'm going to try to do this mostly off the top of my head, so feel free to weigh in if your identified trade partners are different:
1B: The two teams most in need of a first baseman are the two AL West contenders, Texas and LA. After trading Justin Smoak, the Rangers are using Chris Davis at first, and he's been awful. The Angels have Mike Napoli and Jeff Mathis playing first and catching. The Twins could come into play; Justin Morneau is expected back soon, but the after-effects of his concussion have been unpredictable and he could be shut down for longer. None of the NL teams seem to have a need at first base.
C: There figure to be quite a few suitors for John Buck, if teams believe in what he's done so far this year.
I'm not sure I've seen the Yankees linked to catcher rumours, but Francisco Cervelli certainly hasn't been putting up Buck-like numbers. We all know about the Red Sox' catching problems, what with Victor Martinez and Jason Varitek out. However, Martinez may return soon. Gerald Laird has been absolutely atrocious for the Tigers this year, and back-up Alex Avila hasn't been great either. The Rangers have been running Matt Treanor out on a regular basis. Rod Barajas has been bad for the Mets, but his 11 homers might make Omar Minaya look the other way.
RP: The Yankees, Red Sox and Dodgers have been specifically linked to the Jays' relievers on MLBTR in recent days. In order to maintain sanity, I'm not going to examine every member of the other potential buyers' bullpens, but I will note some other teams whose bullpens haven't fared too well: the Phillies (16th by ERA), Reds (20th) and Angels (25th).
Okay, so who looks like a good trade partner after all this parsing of information? Unfortunately, it seems like the best matches come with baggage: the Yanks and Red Sox, who both could use a reliever or two and a catcher, are, obviously, in the AL East. However, I wouldn't put it past AA to make an intra-divisional deal - especially since all of the guys mentioned are free-agents-to-be. The Rangers look like a good partner too, but are limited financially and have already taken on money by acquiring Cliff Lee.
The best trade partner might be the LA teams. The Angels could use Overbay and at least one reliever. Of the players AA can offer, the Dodgers really only need relievers, but Ned Colletti has given up some pretty nice pieces for middling deadline-upgrades over the last few years. So, Bauxites, who do you want from the Angels or Dodgers? Or from any of these other teams? Try to be realistic. No, wait what am I talking about? This is Wild Trade Speculation Day! Be as creative and ridiculous as you want. Think the Jays could pry Evan Longoria for Jason Frasor? Maybe Dan Gilbert for Paul Beeston? Want to propose a 30-team trade? Go ahead! It's WILD TRADE SPECULATION DAY.