I've started using SOAP UI recently to test web services and it's pretty cool, but it's a huge resource hog.
Is there any way to reduce the amount of resources it uses?
It shouldn't be a resource hog, although I've seen it do this before. I leave it running on my PC all week, and a co-worker with a similar machine (dual-core running XP) has to kill it every few hours, otherwise it keeps using CPU. I'd try uninstalling/re-installing. Currently, my instance has been up for 10 days, running a mockservice that I've been hitting very hard (I've sent it thousands of requests). CPU time total (over 10 days) is about an hour and a half, but the "right now" number is about 1%.
There are no popular alternatives, aside from writing your own client in the language of your choice.