Here is a list of the more popular IP PBX Vendors:
1) Digium - the company behind Asterisk - the most popular open source PBX. But even with open source Digium is still going their own way. On its own Asterisk isn't simple to set up and it isn't completely standards-based, but it has taken the IP-PBX market by storm.
2) Pingtel - software-only solutions that are enterprise class and easy to use. Pingtel's software is standards-based and based on SIPfoundry's open source PBX software. You'll need a Linux box to run them on, though.
3) Fonality - low-cost, all-in-one solution (pre-installed Asterisk-based software delivered on a dedicated server). You won't get Fortune 100 level support but you will get enterprise level features.
4) Dialexia - full service provider with solutions from residential up to wide-scale enterprise. Dialexia products are standards-based and Dialexia has been shaking up enterprise telecommunications since 1990, making it a relatively aged revolutionary.
5) Talkswitch - small business phone and PBX systems that are turnkey, based on standards and work with VoIP and existing phone systems.