Asterisk?
Asterisk is a sophisticated software PBX which runs on PCs and not only delivers all of the functions of typical business telephone systems, but also features and capabilities that closed, proprietary systems cannot even dare to dream of. Best of all, you do not have to pay for Asterisk - it is free software.
An open platform
Gone are the days of expensive specialized hardware. Asterisk is "Open Source" so prgrammers can modify it: fix bugs, add features, and more. Asterisk runs on almost any PC (even older 486s) running Linux, Windows XP, FreeBSD, OpenBSD,  & more. It also runs on some routers, and other classes of computers like Sun SPARC. Hardware is entirely generic (even line termination boards, which are available from many vendors).
Asterisk Features/Benefits
Asterisk suppports all standard PBX features (and many that are expensive options on proprietary systems), including trunking, speed-dial, Voicemail (including email notification), IVR, ACD, Conferencing, per-user language prompts, and more.  We tried to fit all of Asterisk's features onto this brochure, but they took up all of both sides, so we'll have to ask you to refer to http://www.asterisk.org/features for the complete list. All information is saved in files on disk or in a database - there is no hidden lock-in (you'll appreciate this if you've ever tried to get all your configuration and CDR's from a proprietary PBX that has suffered a catastrophic hardware failure, or even one that hasn't yet, but might).
Configuration is by editing config files (default) or by menus (see add-ons)