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dBPowerAmp seems to be widely regarded as a good one. I used it back when NIN released Ghosts in flac since I wanted to toss the files on my Zune, and I don't have any complaints. There's a 30 day free trial on it last I checked.
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Well, I assume Winamp. I used to convert other formats from one to another with it like... ten years ago. I assume they haven't removed that.
There's also Mediacoder, but that might be a tad complicated for what you want.
If you're in Linux, SoundConverter works just fine for this, as well.