I use foobar2000, with a few encoders piled in. But I use it to play media so everything fits nicely with me.

For your purpose, I'd get Super.
http://www.erightsoft.com/S6Kg1.html (bottom of the page)

When you first run it, right click the list at the bottom and look at "Output file saving management" - personally I set it to use the source folder.

Choose the output container at the top, you'll want mp3. The encodes typically take a while to start but they get going eventually.
BTW, the output files will get named blahblah.flac.mp3 so feel free to remove the .flac part.

Please note, this program will convert anything to anything and can even resample on the fly - though I have found problems encoding TO Flac (output container needs to be set to Ogg and oddly, renaming the output file from .ogg to .flac will break some players).

Garr, I should have specified, for mac.

http://www.noteburner.com/

Audacity 1.2.6 (Windows or Mac PPC only) with the Lame encoder plug-in will import 16-bit FLAC and export to MP3. Audacity 1.3 beta has universal binary for Mac and enhanced FLAC format handling.