A few things I’ve been looking at:
Glossary:
FXO – Foreign Exchange Office, a device that connects to a POTS provider such as the PSTN.
FXS – Foreign Exchange Station, a device that connects to a phone. Provides power.
voip-info.org, a great reference for all of this stuff.
Asterisk, an open source unix/linux PBX.
Digium, authors of Asterisk and sellers of PCI cards to do POTS and T1/E1 for asterisk. This hardware is not supported on OpenBSD, which lead me to look for alternative hardware.
Sipura SPA 3000, a standalone hardware device that gives you single FXO and FXS ports, plus an ethernet jack. Can do hop-on/hop-off (call the device, connect to an internet telephony service to call outbound). Cheaper than the equivalent PCI card from Digium and is fail-safe; if it loses power or connection to the IP uplink (IPBX / IP telephony service) it reverts to passthrough so the analog extension can access the POTS line.
WiSIP, a cordless WIFI + SIP phone, supported by FreeWorldDialup, Vonage and others. $249.
Vonage, the most well known VOIP service, $29.99/mo for unlimited north american dialing. You must use their provided hardware to connect.
BroadVoice, a more libertarian service. $19.95/mo for unlimited north american dialing. Supports any hardware/software to connect.
FreeWorldDialup, a free VOIP-only (no connection to the PSTN) service. You can call 1800 numbers from FWD and they have peering agreements with a number of VOIP partners; external services available with FWD can be read here.