Mike Zunino was called up by the Mariners and will probably be their starting catcher for the rest of the season. He should provide solid power and I'd rank him somewhere in the 12-15 range for catchers right now.
Alfredo Aceves was called up by the Red Sox to start yesterday and pitched very well but Clay Buchholz should be back at full strength soon enough. I don't think Aceves is worth investing in even if he gets another start.
Josh Lindblom could end up sticking in the Rangers rotation for a while. His next start will come Saturday vs. the Blue Jays. I don't think you want to add him for this particular start but with the right matchup he's a decent play especially with a solid offense behind him.
Justin Maxwell should be back within the next week and can provide power and speed with a low batting average for those in need of an outfielder.
Franklin Gutierrez should return within a couple weeks but he'll only really help in a deep league.
Clay Buchholz is dealing with a neck issue and will not start this weekend but will likely be fine for next week.
Curtis Granderson should return around the all-star break.
Grady Sizemore could end up signing somewhere and being ready to play by the all-star break. If he can get a starting job somewhere he will be worth an add.
George Springer could be called up at some point by the Astros. If you have room on your bench in an AL-only league I would stash him right now. He probably won't hit for a great average but he can help with plenty of HR and SB. I really believe that he will be a very valuable fantasy outfielder for a long time and he is a lesser name than Wil Myers or Oscar Taveras so he's probably still available in most single-season leagues.
Wil Myers should be called up within the next few weeks. He is performing very well at AAA and if he is somehow unowned in your league then right now is the time to make the pick-up.
Michael Pineda seems close to being ready to return and I see no reason not to make a reasonable investment in him.