I have been supporting telephone systems for at least 15 years, and my favorite is definitely the Asterisk Open Source PBX originally created by Mark Spencer of Digium.

Asterisk is a complete PBX in software. We run it on CentOS.
It provides all of the features you would expect from a PBX and more. Asterisk does voice over IP in four protocols, and can inter-operate with almost all standards-based telephony equipment using relatively inexpensive hardware.

Asterisk provides Voicemail services with Directory, Call Conferencing, Interactive Voice Response, Call Queuing, Automated attendant, Fax to Email.

The great thing is that you are only limited by your imagination. You can script pretty much anything that you want with it. You can have it check the weather and read it to you. you can have it access a database and either read (speak) it to a caller or enter new data.

One source that I like for Asterisk unique ideas is the Nerd Vittles blog.

Asterisk is managed via text file, but there are GUI (webpage) tools to make administration easier.

For this, I like FreePBX for the major configurations. We also use Web-MeetMe to manage our conference calls.

To save money on IP (SIP) Phones, we used SIP to Digital Phone adapters from Citel.

This allowed us to keep our old NEC desk phone, but get all of the features that Asterisk provides.

For connectivity, We use a Digium PRI card in the server to connect to the local phone company.

We use a SIP to GSM adapter to make calls to our company cell phones using free cell to cell minutes.
We use Vonage Fussiness to make certain calls less expensive.
We use a “call us” button on our website that lets you call over the internet via an ActiveX control.
We also use “Soft Phones” to allow our traveling users to make and receive calls on their laptops.
We are testing a SIP to Skype setup that will allow use to us Skype from our desk phones.

Asterisk can also be used as a home PBX.

The main source for Asterisk information is at http://www.voip-info.org/wiki-Asterisk .