I've used CDex to do this in the past.
I'll throw in the obligatory "I wouldn't do this" if you are concerned about the quality of the files. Going from one lossy format to another is only going to sound worse.
Having said that, I do this all the time. I'm not a audiophile at all, and I'll convert 256+ kbps MP3s to 128k WMAs just to maximize space on my MP3 player for use in the gym. I still keep the higher bitrate files for my collection, but when I'm working out or casually listening at work, high quality isn't a big deal to me.