When I have a choice (not very often when working with a company) I use FogBugz. It's a bit pricey compared to FREE, but now that they have a hosted version, it's great if you need to track bugs with a team.

The simplicity and approach of FogBugz is fantastic. Really worth a look if you have the cash to afford it. That being said - they do have provide it for free if you only need 2 accounts. See: http://discuss.joelonsoftware.com/default.asp?biz.5.482288