Well, I'm using FingerPrint - check HERE - not free but works fine for me on my Cannon Pixma via Wi-Fi (need the program on your computer) - has printed for me from a number of different iPad apps w/o a problem - recommended by Tim.
Now, there are numerous threads on printing from the iPad, so if not already done, do some searching - other options exist and have been recommended by others - good luck in your choice(s) -