The short answer is: Asterisk
The long answer: ...
I use Asterisk with 1 Danish VoIP, 1 UK VoIP and 1 US VoIP provider as well as 1 regular PSTN line. The dialprefix tells Asterisk which way to go :) Incoming calls go either direct to an extension or into a ACD where all phones ring. Unanswered calls go to VoiceMail. Conferences can be hosted if family meetings are required.
Asterisk can do regular FXO/FXS using hardware from Digium (inventors site), ISDN-BRI from various sources or T1/E1 equipment from various sources.
Protocols are limited to OSP, Enum, Dundi, ADSI, SIP, H.323, MGCP, SCCP (Cisco Call Manager), IAX. Other projects (there are many) available for install are: Nortel native protocol support, FAX send/receive and SS7.
The smallest system for Asterisk reported so far is an X-Box, I've heard of people working on Sony PS/2 implementation and also builds ment for install on LinkSys WBR's so basically anything bigger goes, AMD, Intel or PowerPC running some form of Unix; Linux, BSD, OS X, Solaris ... (pending hardware support)
Anyway, for my last homesystem, I downloaded the Asterisk@Home ISO, made a CD, put it in the pc and powered up. The @home project will format the disk, install CentOS-3 (stripped down RedHat Enterprise), install Asterisk PLUS additional tools so in fact you will (almost) never need to telnet to the box, you can do mostly everything from a browser. I had it up and running in a usable state in a couple of hours.
/Soren