[Serusers] Softphone that has STUN or built-in NAT support?

I thought this bit would be easy, ask a few friends to register on my server and test IM and voice calls. It turns out each and everyone of them has NAT. A quick run around google tells me if I can find a softphone that supports STUN, or can be configured to use the IP address of the NAT gateway and I'll be set. Unfortunately I cannot find any *free* packages that offer these features. Has anyone come across I hate to think I'll have to acquire Cisco phones for my friends to continue testing. While I'm at it, I also was looking to find the least expensive SIP phone on the market. All I've found so far are links geared to large enterprises or OEMs, nothing a tinkerer could buy. Thanks, Dan