I bought the Ortlieb OfficeBag last week. It comes in Medium and Large. I got the Large. It fits a laptop plus assorted accoutrements (lunch, office clothing, lights, etc). I got it for $95 on sale at REI.

So far i like it. Good workmanship, waterproof (so they say, haven't had a chance to experience it myself). Easy on/off the rack. Stable. I don't notice the extra weight hanging on one side when biking.
It does require a laptop to be padded. I use the one that my company gave me with my laptop. Simple affair, but does do the job. A lot like what ax0n describes in post #2

If you just want space for a laptop, you may consider the Medium size. Nearly identical, but narrower.
Duppie