All of the UPS transformers I've seen are basically 120V in, and two center-tapped ~14V windings. One is low-current for control electronics, and one high-current one for the battery supply. You certainly could use this for some interesting power-supply projects. The obvious one is an LM317 supply. You'll be limited to 14V or so output, unless you want to do some boost-converting, but if you choose the right parts, you can get several amps at that 14V. I don't think you'll get 50A, though. That would be drawing something like 7A from the primary, which seems a bit much for a transformer that size. I've got one from a similar size UPS, and I think the most I've ever got from it is 10A or so. Which is still a lot of current!
A linear converter is easy, and an older unit may have ventilation built in you can use for heatsinking. A switching supply would be a neat learning project, as well. Some high-powered USB ports is certainly feasible, especially if your transformer is center-tapped. Just stick some 5V regulators on one of the ~6V taps and you're good to go.