freeswitch.org
If you are using Asterisk now, or are planning on building any kind of VoIP-related system, I recommend you try Freeswitch, it is a lot simpler to setup, configure and use in general, and has a great online community of users and developers.
If you really want to build a phone system, check if you need a PBX at all. There's a lot of OS software that handles telephony. There's opensips or yxa if you don't need PBX features, only proxy. From PBXes there are asterisk, freeswitch, yate (some people say that headers from grand central indicate google uses it) and some other less popular ones.
This article is mostly about - asterisk crashes, and fs doesn't ;) Don't go for the popular ones - check which features you need first and then decide.
Asterisk is impressive that it gives a customisable PBS of sorts, but peel away the hood and it's all duct tape and bubble gum, terrible documentation left in the hands of a run-down unmaintained wiki and extension libraries unmaintained since 2004 full of out of date warnings about which bits are and aren't working in various versions of Asterisk.
It works, but it's not pleasant or fun. Is Freeswitch better?
At my company we are currently using Asterisk, but are in the process of switching to Freeswitch. Everything seems very well thought out, and in my opinion, it is way simpler than Asterisk. There are many modules for a lot of different functionality, there's support for javascript, lua, perl, java, C, C#, ruby, python, php, a skype gateway, and many more.
It is more stable and its routing does look more "sane" as far as SIP goes. If I was starting from scratch, I'd choose FS rather than Asterisk. Then again, I would choose some other systems before FS.
I started to set up a FreeSWITCH system, figuring if it was a clean redesign I might as well get on board now. Then I discovered it used XML in config files, which made me question the developers credentials and I never looked at it again.