P•B•Xes is an online virtual PBX that has many features of a fully functioning PBX. For a nominal monthly fee, it can be extended to have all/most of the features that anyone could ever need. There are currently four service level offerings: Free, Soho, Premium and PBXes Pro.
I spent some time playing with this tonight and have configured four extensions, a ring group, inbound call routing, music on hold and linked the PBX into my Google Voice phone number. Not too shabby for a free service. An upgrade to the soho bundle enables a few more interesting features like voice menus (Digital Receptionist), Google Voice/Talk Trunks and 5 more extensions for a total of 10. The service, however, gets much more interesting at the $15/month Premium account which will add Call Queues, Call Recording, Faxmail as Email, Follow Me, ENUM, Phone Book as well as bring your total minutes up to 5000/month.
If you have an interest in playing with a PBX, then this will provide hours of fun. For a small business, this could be an interesting, cost effective method of keeping all your employees in touch, giving employees unified messaging (email, voice mail, faxes all in one mailbox) and linking multiple offices (even remote offices) together for ease of internal communication as well as presenting a single point of access to your customers.
If you want to chat about this, I’m always willing to talk about telephones (much the the chagrin of many of my friends 
). Feel free to contact me or request a free consultation.