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