From time to time, someone will ask me for help buying a bike to ride around town.  While many bike companies make great "around town" bikes, there are a few accessories I would focus on over the bike itself that make up a great commuter.  In order from most importance to least

Brakes

I'm shocked that I needed to mention this one.  Every now and then I see it near my office: Skinny-jeans hipster on the fixed-gear track bike with the neon green deep-v rims. (No Brakes).  Enter cargo-shorts and flip-flops surfer dude on the longboard.  They both swerve the same way and then... smash.  

When riding around traffic and pedestrians things can happen quickly. Sometimes you need to stop.  And trust me: I've tried it; that clean looking fixxie you're riding... the ladies don't even notice it.

Lights

When cruising around town, it's important to be seen by cars.  Being able to see is also important, but I'm much more worried about the cars.  I'm of the philosophy you can't have enough flashing lights on your bike, bag, everywhere.  Passive lighting (i.e., reflectors) are also great. I'm a huge fan of my Fiks reflective rim strips, and reflective stickers on my bike.  As far as headlights go: there is a huge market that deserves a separate post, or you could build your own.

Fenders

The wet back/butt stripe is not cool for anyone.  If you live anywhere where precipitation occurs, fenders are a must.  Even if you're not riding in the rain: riding after a rain, during a snow melt, or through a puddle can all fling mud onto your pants/shoes and back.  Not only do fenders protect you, but they also protect your bike components from the wet road grime that is flung up from your tires and wreaks havoc on your drivetrain and headset.

Note: many road bike frames will have little to no clearance for fenders, and they also may lack the eyelet's to install the fender hardware.  They do make special adapters if your bike lacks the right attachments.

Panniers

If you want to start hauling stuff on your bike, you'll need some panniers. I have two sets of panniers I really like.  The Avenir Metro 2.0 Shopping Pannier are a great cheap set of panniers that will easily hold a weeks worth of grocreys, or a wide assortment of other things I need to haul around town.  My Ortlieb - Back-Roller Classic Panniers are great for carrying a change of clothes and my laptop to work (espically when the weather is bad).

Gears

I've done hour-long commutes on my bikes, and live in an area where there are some hills.  I always appreciate having a few gears to select from, however I don't think you need 20-30 speeds to ride around town.  Gears require upkeep and maintenance  and I think the best soluiton is an internally geared hub.  Apart from that, running 1 speed in the front and 8-9 speeds in the back is a nice way to have a low maintenance geared setup.

On the opposite end of the spectrum: I've tried the single-speed gig.  It's not so bad until you encounter a steep hill or are riding home with a heavy load of grocreys.  Then you wish you had some speeds.

Cost

Theft is an unfortunate reality in bike commuting.  You could have a sweet riding commuter bike, with a carbon belt drive, internal geared hub, sweet fenders and head lights from a Lexus. Then you ride it downtown to meet buddies for happy hour. One thing turns to another and you end up catching a ride home. Return to pick up your bike the following day, and see what's leftover.

If you're storing your bike inside, this isn't as big of a problem, and usually locking it up outside a restaurant while you eat dinner is fine, but you never know.  I've had $30 lights taken from my bike... and I've also forgotten to lock my bike all day with no consequence   A great commuter is a sweet ride that doesn't scream "steal me" and doesn't put you out too much if somebody does steal it.

The bikes.

Now that i've outlined some accesorries, i'll give my top picks for commuter bikes.  I think cyclo-cross ibkes makes for amazing commuters.  they're light and fast like road-bikes, but can handle knobbier tires and have the clearance for fenders so they do great in all-weather riding.  They usually have eyeletts for racks.

The english style 3-speed is a sharp looking bike that is perfect for many urbanites.  they offer an upright posistion, and are usually pre-mounted with fendors, a rack, internally geared hubs, and chain gaurds.  They're maybe a bit heavy to commute between cities, they're a great option to ride a couple blocks downtown on.  BikesDirect.com usually has some great deals on some sharp looking bikes.

An old hardtail mountain bike will have a comfortable up-right position  and low value. If you use one of these, you may consider putting thin, slick tires on it to reduce the rolling resistance.  Old road-bikes will do too, although they sometimes don't do so well with fenders and can have an uncomfortable crouched-over position.

Of course, my dream commuter bike would look like this: