I use Audacity (free, open-source) and Adobe Audition ($$$). With Audacity, you will also need LAME to do the MP3 conversion.
If I were to record a lecture or presentation I would probably just use my Sansa MP3 player to do the recording and then edit/convert the file with Audacity or Audition. But if you have your laptop with you anyway, you could record directly to it. If your laptop doesn't have a built-in mic or if the built-in mic is not suitable you might need an external mic.