Lot of great suggestions, and I may be repeating some of them. Here's my list of (not in any particular order, except the first):

- Wear a helmet.

- Intense-beam lights front and back, plus your stock reflectors.

- anti-puncture/pinchflat tires (Bontrager Hardcase Selects run relatively smooth). Best upgrade I put on my bike as I was getting flats halfway through my trip because of small bits of glass or metal.

- Keep your main krypto- or heavy lock at work (locked on the rack) and carry a small, lightweight coffee shop lock.

- If you're a rainy-day or all-season commuter, you need to get fenders and cycling rain gear. I say cycling rain gear because it's made to be breathable and you don't build up as much sweat underneath. My first rain-day commute I used an old rain coat and by the time I got to work, I was more soaked from sweat than rain. Keep your feet dry with waterproof socks or booties that go over you shoes. If you wear glasses, find your old pair and spray Rain-X on them and use them specifically for your commute.

- Carry some cash and a cell phone on you.

- Keep some spare clothing and shoes at work.

- Carry a small/lightweight bike multi-tool with you.

- Keep a spare pump at work.

Hope this helps!