I find my legs get the most workout just as Gadd has said. Consequently I think excersises that focus on them are good such as squats and lunges.
Also, something I would not have thought about was at 1st my neck would suffer and I realized that ukemi technique required protecting my head with chin tucking, etc. It took a while to develop some neck strength. This helps especially with any kind of ukemi where you fall directly backwards to the mat.
Abdomen wortkouts are good because it really helps with back strength. I find its not the fall itself that strains my back but some of the postures where you arc and stretch the back doing ukemi. Lastly stamina training such as running is useful as I find I need it for randori, etc.