I keep forgetting my research with this stuff, so here we go.

Free incoming calls (NY number, currently): stanaphone.com

Free outgoing calls (requres $12 in account): voipbuster.com

Both allow you to use SIP devices, IIRC.

NSLU2
Nerd Vittles
Aastra VOIP Phone
PBX in a Flash Setup
Asterisk on NSLU2
Phones
Trixbox