Too many posts to search and see if this has already been mentioned, but I would add an mp3 player and speaker to the must-have list. I keep a $9 San Disk 4 gig that runs on a AAA battery and a speaker I bought on eBay mounted to my handlebars. Both are easily removable when I arrive at work.

Music makes the commute go so much faster and can generate a lot of energy for the work day ahead. My player also has a radio in it, so I can catch up on the news in the morning.