I use something from Lowes plumbing department. It costs less than the others that I have seen.
http://www.oatey.com/products/putty-caulks-and-water-barriers/epoxy-putty/fix-it-stick-epoxy-putty
Foster & Smith has a 4 oz tube of green colored epoxy at a good price. It changes to white after 30 minutes.
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=3961
I find that if you put a coat of super glue on the 2 surfaces being joined, then sandwich the epoxy between that, it sticks pretty good. In other words, when gluing a frag plug on the underwater rock, put superglue on the plug, then the epoxy on top of that glue, and then superglue on the bottom of epoxy where it will stick to the rock. And it wants to be shoved in a hole, or crack so that it locks in to the rock. I prefer to peg my corals, but I still use this method for attaching corals to the pegs.