If you have a LOT of cassettes you might be interested in this:
http://www.cyberguys.com/templates/SearchDetail.asp?productID=10172It fits into an empty 5.25" drive bay.
I've never used one so I can't speak from experience.
The following is my super cheap method:
Cassette Player
audio cable with 3.5mm male connectors on both ends.
plug one end of the cable into the cassette player's line out or headphone jack and the other into the microphone or line in jack on the computer.
Record the audio with your preferred free software.
This is a rather time consuming method because you will have to manually stop the recording between songs. I've copied a few individual songs this way but not an entire cassette tape collection.