Thanks everyone! 
Gear used and how I took the shots:
#1: 20D + 100-400 + 36mm Kenko macro tube, handheld
#2: 20D + 100/2.8 macro + Sigma 1.4x teleconverter, handheld
#3: 1D Mark II + 500/4 + 1.4x II + 36mm Kenko macro tube, from a tripod
#4: same as #3, macro tube is to shorten the minimum focusing distance
For the first two shots I crawled around on my belly and slowly approched the frogs, they didn't seem to mind as long as I was making slow movements. The light was direct sunlight, I used ISO 400 or 800, and I could steady my arms on the ground so the camera shake wasn't too bad.
The third and fourth shots are from another day when I was out to photograph birds, so very different gear. I was sitting behind my tripod and decided to try the frogs as the birding was very slow. The frogs and toads in the water were actually more difficult to catch than the ones on land, they were moving around quite a bit and were also very watchful. Make some movement in their direction and they'd dive. I cleaned up the water in #4 in post processing to remove some floating debris but it was quite clean to start with.