Ideal lens and speedlight for party pictures?

I have a Nikon D5000 and I'm interested in taking pictures at house parties (dark areas with tons of people drinking/smoking/dancing etc.) and I was wondering what lens and flash would be best for taking pictures in this enviroment!

You'll want a fast lens. I would suggest the Nikon 35mm f1.8 which costs about $200 (US) or around $250 (CAD). As for flash, if you really want one instead of your built in flash, any external one will do. Depends on what you want to achieve in your pictures.

The 35mm 1.8 is a good lens.

For a flash, look at buying a bracket, sync cord and diffuser. It makes the shadows a lot softer. The best flash for budget/performance is the SB-600, around 200.00. The SB-900 is about twice that. If it's really dark, look at the SC-29 sync cord.

Lumiquest makes some good diffusers, and you can make a bracket yourself for about 15.00 with a piece of Home Depot aluminum bar stock, or about 50-100 to buy a Stroboframe.

Here's a shot taken with slow rear sync, bracket and diffuser:

http://www.cgipix.com/Escondido_Skate_Park_Professional_Rollerblades_Photography.htm

Fast and wide, tamron's 17-50 uis a nice budget choice.Sb-700 on it and a lot of practice.