This is something I looked at a while a go and it seems there are a few ways to go about it.

If you are lucky enough to be in the right place at the right time (e.g. Tibet over the last couple of weeks), you might be able to sell your photos to news agencies.

If you are skilled enough you could look at selling your photos as art. Set up your own site or maybe use eBay? Buyers will most likely be looking to buy prints (maybe framed), but there are online services that will do that for you and ship directly to the customer.

The easiest way though might be to get your images on some of the stock photo sites and get paid each time your photo is used. Here's a few of the sites that will accept your photographs:

http://www.istockphoto.com
http://www.stockxpert.com
http://www.shutterpoint.com
http://www.photostockplus.com

There's some good info on making money from photography on this site: http://www.photosecrets.com/tips.html#sell

Anyone else got any tips on freelance photography?