Welcome, Lizzies! Tough question, and one where I can only give you a bit of guesswork since there aren't that many cold, hard facts out there. Speculation is always fun, though, so here goes:
Esteban Yan. A trade away from Tampa Bay is a distinct possibility, but there's no short list of potential takers yet.
Bartolo Colon. It's still possible that the Reds will be able to pick him up.
The entire Tigers roster is available in trades. Jeff Weaver might be a good possibility.
The Blue Jays would love to move an outfielder, but it doesn't seem likely that this will happen soon.
The Yankees will most likely make a deal of some kind, potentially dealing prospects. I doubt Drew Henson will be going anywhere, but Brandon Claussen and Erick Almonte seem possible. The Yankees may also be ready to shop Nick Johnson.
Sindney Ponson could also find himself on the move.
Again, this is mainly speculation. Most likely, we'll be surprised by most of the trades that actually happen, but these look like good scenarios to keep an eye on right now.
Also, watch if any AL team shows interest in Jack Cust.
As to Byrd, my feeling is he'll only be called up relatively late in the season unless an injury opens up a spot. There's no timetable yet, at any rate, and Giambi gives the Phillies a bit of roster flexibility.