Sony SX712D digital voice recorder with Dragon NaturallySpeaking voice-to-print software
At $199.99, this is the most expensive item in our roundup, but it'll be worth it for those days when you just can't pay attention in class. The recorder has a two-way stereo microphone and a built-in battery (rechargeable via USB) that offers 25 hours of recording time. Transfer the recordings to your PC and use the included Dragon voice-recognition software to convert it to text.
Two things to note: One, the software doesn't work on Macs (though you can still transfer the recordings to your computer); and two, some professors require that you get permission before recording their lectures -- it's best to ask first.