There's nothing wrong with a man carrying a bag with one strap. Here are a few of my favorite options.
The Timbuktu Command Oxford Nylon Fabric messenger bag is a good example of relatively affordable utility. It's the least-expensive bag on this list ($130) and includes a padded, zippered compartment on the back of the bag for easily removing and replacing your laptop--very handy for the frequent flier.
If you've got the cash to spare, few vendors make messenger bags as tough as Chrome. My personal favorite is the Citizen Limited Edition Fowl Mood, but then I have a weakness for owls. You'll get plenty of carrying space and Chrome's signature seat-buckle closure for $195.
If a briefcase replacement item is more your style, check out Jack Spade's stylin' bags. The Dart Messenger Bag puts many women's handbags to shame with its soft leather construction and sleek looks. Of course, it'll cost you...$350, to be exact.