Is there a decent freeware program that can convert a cassette tape to mp3?

No

Yes, Audacity is a great one. You need the ‘lame’ mp3 converter to make Audacity work with mp3, but it is a great program for recording and some editing. Simply run a cable from your cassette player’s line out to your computer sound card’s line in, then set Audacity’s recording input to line in, and off you go!

This is really easy to do. As @FireMadeFlesh says, all you need to do is plug your cassette player to your computer via a line-out (from cassette) to line-in (on computer), then use Audacity to record as you press play on your tape. You will be left with a WAV file, which is an uncompressed mp3 file, which is what you want when recording (only use mp3 when you are done processing, as you will lose some editing data).

@FireMadeFlesh @dynamicduo Thanks. A .wav file is fine. I have an editing program that will snip it down, and I have another program that will compress it to mp3.