NerdVittles has posted a complete, step-by-step how-to on installing and configuring Asterisk for the newbie. Asterisk is a PBX (Private Branch Exchange) in software. It runs on Linux, BSD, and OS X and provides a wealth of phone options for your home or small business. Essentially a PBX is a private telephone switch, so making a digital version within *NIX allows you to completely control what happens to phone calls as they come in. Asterisk, being open source, has a ton of cool modules for use, for free.

Asterisk can handle "normal" calls, but it is particularly tuned to VoIP, and handles a wide variety of codecs. I like the features a small business would need, such as hold music, unlimited conference rooms, and a great voicemail system.