PBX Vs. Key Switch

What are some popular PBX or Key/hybrid system companies that offer telephone solutions for small businesses (15 – 50 employees)?

I know of Toshiba, NEC, Avaya and Nortel (Meridian). Key features are: 4-way conferencing (3 outside lines/1 inside [host]) or more, multilines, simple, but functional voice mail features that are accessible from either a station or through DID and ACD queues (where by a phone can receives ACD queue calls and maybe able to transfer or receive calls outside the queue. We use an NEC Elite IPK and this system does not offer any of these features.

Thanks