Don't forget a shotgun (for the zombies).

I don't think I've ever spent $500 on cycling in my life. It's possible to get going on a lot less, including the initial purchase of the bike. Gloves are quite useful if you come off as they offer a bit of hand protection when you put your hand out to break your fall, as well as keeping the wind off. CO2 inflators are convenient, but only good for one use, whereas a simple pump can be clipped to the frame and used forever. Likewise, if you don't mind stopping for a drink, a specialist water bottle and cage aren't necessary (I originally bought mine to hold the batteries for my lights.) I've also never worn cycling shorts- if the standard saddle is uncomfortable, it doesn't fit properly. A comfortable, practical saddle will allow cycling in regular clothes.

Without knowing how long or where your commute is, it's pretty hard to say what is "essential" other than the bike, and some way to carry stuff.

Beyond that, we need more info. But, in general:

1) A helmet, if you are a helmet believer.
2) A lock - otherwise your bike is gone and everything else is irrelevant.
3) Lights - if you ride in urban areas, a front and rear blinky, to be seen day and night. If you are absolutely sure you'll never ride at night (as in, you'll take a taxi or bus home), or you ride in very well-lit areas, that would suffice. If you are riding at night and there is not enough light to see by, you need a good "to-see" headlight also. The Planet Bike Blaze is a good bare-minimum light, and the sky's the limit from there.
4) Tools and such - again, if you are riding in an area with public transit or taxi service available as a fall-back, I'd go very light on this stuff. The mini tool is always useful, and I'd buy really good flat-proof tires instead of hauling a pump and spare tube with me every day. However, if your commute is 20 hilly miles out in the country, you'll want fast tires, and ability to get yourself home no matter what happens.
5) Fenders - If you are in Phoenix, fenders are less important than if you are in Seattle. Also, you may decide you don't want to commute in the rain, and employ alternate transportation those days.
6) Stuff hauling - you might want to try the basic messenger bag or backpack for a while, and then see how you might want to change your hauling strategy, if at all, once you have a better idea of your needs.

The lists I've seen are pretty complete, however I'd toss in these items.

Sharpie pen (velcroed to the stem) for writing down license plate numbers as needed.
Conspicuity tape ($7USD) to add reflective touches to the bike (as well as several friends' bikes).
Lightweights for wheels, to add reflective touches to the bike (watch this looping video)
Camera for documenting the good and the bad you see on your way.
Good rain gear, riding in the rain can be fun, but being cold and wet sucks.
Cell Phone in the event of an emergency.
Bike ID kit, being ripped off sucks. ID kits can help deter thieves, and get your ride back if it's stolen.
A bike computer for fun (and you can join us on the The 2011 Commuting Mileage Thread)

You're absolutely right, Monster Pete. It is possible to do it for next to nothing. However, I like new, high quality gear. I also bike commute 5,000-6,000 miles per year in all weather conditions and all seasons, and don't have the time or desire to hunt for freebies and the like. Someone who rides in the city 2 miles each way can get away with much less gear. I guess the best answer to the OP's question is that it depends on your needs and inclinations.

I'd put a high visibility shirt near the top of the list. You don't need something expensive from a bike shop... just a ANSI orange or green shirt like the construction workers wear.

Someone mentioned above that if you have a gas stations nearby with free air, that this could be a substitute for having a pump. Just a word of caution that this is only an option if your bike has Schrader valves. I've yet to encounter a gas station air chuck that would handle Presta valves.

I know this because years ago when I purchased a bike the salesman asked me if I needed a floor pump. I told him no, as I had an air compressor at home and just figured I'd use that to pump up my tires. Later when I discovered that my tires had this weird air valve I'd never seen before, I was back at the LBS sheepishly asking to see their floor pumps.

The 7.3 looks to be the better choice in terms of componentry. My own personal preferences for gear/accessories can all be had from https://www.bikeisland.com w/no shipping charges. Not a shill for this site. The equipment and service are top-notch. Fenders, lights(for darkness riding at least 10w halogen for the front), multi-tool(bikeisland has a Park MTB-3 which has a built-in chaintool), tube(s), tire cleat, tire levers, patch-kit, rack, panniers or trunk bag, frame pump(Topeak Road Morph w/gauge) and a wb cage. If you're going to ride in darkness reflective tape and clothing are requisites as well as blinking taillights.

Looking at my commuter/utility bike as I type this my rig has all the aforementioned accessories and doesn't look overloaded or busy. In addition to a night light I run a blinking daylight 'be seen' light as well. Side reflectors in my spokes along w/reflective sidewall tires.

My commute's pretty long so it's necessary for me to equip my commuter as if I'm doing a light tour everyday. Your needs may not be that extensive, but getting good quality gear is a good habit to develop. Over the last 5 years of cycle-commuting I've had 2 full-stop road emergencies. Both were flats. No mechanicals. The better part of stress free commuting is consistant maintenance. Haunt the Mechanics sub-forum. Ask questions. They're the best and are very generous w/their knowledge. "Better to know and not need than to need and not know." B.B. King

So, do your homework. Have fun!

Wow. LOT of opinions on here. Many of the things listed I wouldn't call anything close to essential. In my opinion, like a few others have said, you have a bike, and the desire to ride. Everything else, you'll figure out.

For example:

I was riding w/ a bag & a lot of books- didn't like the weight on my back, so I got a rack & pannier.

I was riding w/out any patch kit or repair tools, then some friends got flats & I decided I'd rather be prepared, so I got what I needed.

I always had a helmet & lights, but one rainy night I went over some railroad tracks, flew off the bike, and landed in traffic. My helmet hit the pavement, instead of my head, and the lights prevented me from getting hit by cars. Now I don't even question using them.

I didn't like riding on platform pedals, so I got straps & cages. Didn't like those either, so I got clipless, and I love them.

Just roll with it (no pun intended), you'll figure out what you need. Good luck and have fun!

Any lbs will have a brass presta to schrader adaptor and some frame pumps come equipped w/them.