Here is the USB Tape Express, a device that let you convert muisc on your old cassette tapes to digital MP3 files. The portable device uses USB interface and is compatible with Windows XP/Vista and Max OS X. It can be powered by two AA batteries or via USB connection.
The USB Tape Express does not only conversion. It has audio output jack for you to listen to cassette tapes. It comes with EZ Tape Converter software. It is priced at $81.