What are the essentials for event photography?

I have a Nikon D80 right now with a 18-135mm lens. Any suggestions for lens/flash/filters? I want to do an internship with a local music scence newspaper. Also, how do people get crisp pics lowlight? They obviously dont use a tripod...

Is that your only lens? If so, I'm afraid you've got a lot of equipment to get. Event photographers use fast lenses. Something along the lines of a 24-70mm f/2.8 and 70-200mm f/2.8 are the zoom lenses you definitely need to have. Also consider getting really fast prime lens such as 50mm f/1.4 or 85mm f/1.4. As far as flashes go, I would only consider the SB-600, SB-800, or SB-900. Depending on the event, I'd use them sparingly.

As far as filters are concerned, you probably don't need any. I usually only recommend three filters: circular polarizer, neutral density, and UV Haze. (UV filters for protection of the front element is just about as good as a wax on a car protecting a car from damage. Might help in wet situations, but impact is still impact. A strong hood does more for impact protection than a UV filter does.)

I've just mentioned a lens hood, but let me mention it again. A good strong lens hood does wonders for protecting the lens... especially for event photographers. ;) You should have the corresponding lens hood for every lens you have. Don't go cheap and get the rubber ones. They are fine for preventing glare, but do nothing for protection. When photographing events, even if I am careful, people are always running into my lens.

Also if shooting events professionally or semi-professionally, I would recommend getting some kind of liability insurance. Even though you may have every right to be there and shooting, some bozo may not like that and try to make you homeless. Even if your internship provides some protection, I would still make sure you've got some kind of protection. (You can join the PPA at a reduced rate for a couple of years and they will protect you.)

Now for the final part of your question... Crisp shots in lowlight without a tripod are achieved by using the lowest possible ISO you can + fastest lens you can for the shot + proper holding technique. Also, this is one case where the camera sometimes does matter. FX sensor cameras perform better in low light. When you're ready to upgrade you D80, you might want to look at the D700.