Fusion’s Decoupage and Transfer Gel is a fantastic product. You can use it to accomplish a variety of techniques.
Fusion’s Decoupage and Transfer Gel is my favorite product to get a smooth, bubble-free decoupaged finish. Apply two light coats of Transfer Gel to your surface and allow to dry. Place your paper over the the dried Transfer Gel and Iron with a dry iron on hot heat. The heat melts the transfer gel and fuses the paper, creating a smooth finish. You can read the full tutorial here.
For those of you who have played with Mod Podge, Transfer Gel is different in that it provides a means of transferring the actual image, so your end result is just a picture with little to no paper remaining on the piece. You can find a full tutorial here.
Decoupage onto Glass, metal, wood- you name it!