I've been distracted by VoIP telephony for the last little while. I didn't really know that there were so many open source projects for VoIP. What seems really cool, is to get one of these (pictured over on the right) and one of these and make a little home telephony project. The latter allows for 1 incoming/outgoing analog line (an FXO) and up to 4 analog phones plugged in on the prior. More of those cards can be added if you want more analog phones plugged into the system. Then, you get an open source PBX and telephony toolkit, like Asterisk to do all of the PBXing and voice mail. That way, among many other features that Asterisk has, you'd be able to call other phones in the house, and calls that a certain phone is on can't be eavesdropped upon without purposefully conferencing someone in. Then, if you want to get VoIP phones, you can add those into the system by just plugging them into the LAN and adding them to the Asterisk config. You can also use software phones like iaxComm on computers if you want, to interface to your analog line and other phones in the house. I'm sure you could set up phone numbers that call other IP phones out on the Internet if you wanted to, too. iChat is, under the hood, a SIP client itself for voice and video, but I don't know about getting it to work with Asterisk, as it appears to only open up the SIP ports if a call is being initiated through AIM. It sure would be neat if Apple made it a full-fledged SIP client, though.
Some of the cool features of Asterisk are that it can call route by callerid. For example, if someone calls that you don't ever want to talk to, you can give them a busy signal. If someone calls that doesn't have a callerID number, you can give them a "special information tone" (you know, like the tone you hear when a phone is disconnected) and that will often cause autodialers to disconnect. You can also have it ring a certain phone, and if no one answers for a while, it can ring another phone or a set of phones. If no one answers, you can have it answer and say no one is available, and ask for a voicemail that will go to a public mailbox, or whatever other mailbox. You can have e-mail notifications that someone left a message in your voicemail box. You can access your voicemail box from the web. I'm particularly interested in mailing a voice mail to my e-mail box. That'd be cool. Another cool thing to do, is to have the system answer after, say, 9pm, and have it say, "Sorry, it's after 9. Please leave a message." Then, give only members of the family and close friends a secret extension they can enter if it's an emergency.