iMic: A USB Audio interface. Works for both Mac & Windows for $40. Can be used to record analog as well, but I use it as a way to have a second sound card for my Mac.
DJ 1800: a relatively cheap program to let you cue up music. ($80 - others cost several hundred dollars). I use it all the time to make sure I have the mood / etc right. You can download it for free, but it will automatically shut down after 30 minutes.
If you use iTunes (I do) and own a Mac, you can use bpmWidget to figure and set the BPM (beats per minute) for your currently selected song. This is not automated in any way, all manual. I used to use BPM Inspector to figure and set the BPM (beats per minute) but that stopped working when I upgraded to 10.5. I'm not sure if it was my system or not, but I used this instead.