If you're using a Mac, in your iTunes folder on your hard drive, there should be a folder that says something like mobile apps or whatnot. Assuming you didn't delete the apps off your hard drive, all the apps you've purchased through that computer, along with the transferred one, should be there. Back on iTunes, go to "Add to Library" and direct to that particular folder and you should see them back on your computer.
I imagine the procedure will be similar on a Windows machine, but I don't use one at home, so I don't know for certain.
EDIT - It should work similarly for other things from iTunes (songs, TV shows, etc.), you'd just need to find the appropriate folder(s).