ANONYMOUS  Absolutely easy, simple and effective for it's purpose.

Because it is a widget it is alway ready, allowing to set the BPM of the current song in a few seconds.  
(Version 0.2)

ROLLANDBURN  1. Install widget

2. Play song

3. Hit F12

4. Click mouse to beat 10-15 times

5. Click 'Set'

6. Notice iTunes now has updated BPM column.

Simple pimple....to automate this process instead of doing it manually is the next best thing. Maybe I would prefer to hit the space bar instead of the mouse button but, meh, it works.  
(Version 1.0)