This topic has been covered thousands of times. Unfortunately the forum's search is the worst I have ever used. Check out these threads for some good ideas and input.

http://www.google.com/search?hl=en&c...et&btnG=Search

I personally prefer a backpack over a messenger bag. I commuted with a Chrome Metropolis exclusively for over a year, and it got annoying if I had to walk anywhere with it (weighing down one shoulder). You need to be more careful about how you pack a messenger bag too; otherwise, you will have things jabbing you in the back the whole ride. I agree that a weatherproof bag is the only way to go when you're commuting on a bike. My current bag of choice is the Ortlieb Flight which I reviewed on the forums a while ago. The other weatherproof backpacks I've heard good stuff about are from Reload, Seal Line, and Freight Baggage.

If you truly have your heart set on a messenger bag, then there are many many companies that make them. Some of the more popular companies are Chrome, Reload, Under The Weather, Timbuk2 (questionable quality), Crumpler (Are these totally weatherproof?), Pac Designs, Seagull (relatively new company, but Ohio based), Bailey Works, Freight Baggage, Bagaboo, Banjo Brothers, and Chicago Wig, to name a few.

Somebody may recommend panniers, but I don't like that solution. There are panniers that can hold a lot, and they are easy to detach, but most of the larger ones won't be very easy to transport off the bike. If I'm using panniers it's because I have too much stuff to fit in my messenger bag or backpack. I can't leave them on my bike because somebody could walk off with them and my stuff, and carrying them around with a shoulder strap is a pain.