Cassette tapes aren't exactly the highest quality audio, but if you have a few rare tracks that you can't buy on CD (or a few homemade cassettes you want to back up for posterity), CNET shows how to easily record them to an MP3 file with Audacity.
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The process isn't that complicated -- all you really need is a tape recorder with RCA outputs and a computer running the free Audacity audio editor. However, just plugging it in and hitting record isn't going to get you great audio, and the guide has a few tips (and a great video walkthrough) for getting the best possible final product, from cassette to iTunes. Hit the link to check it out.