Free World Dialup
FWD offers free SIP accounts, conferencing, and calling to PSTN "800" numbers. http://www.fwd.pulver.com/
SIPphone
SIPphone.com offers free SIP accounts and sells prepaid calling to the PSTN. http://www.sipphone.com/
We have had some discussions about peering with some of these services to allow easy dialing to and from MIT phones.