I've been using this virtual phone system from RingCentral for about a month now. So far, I really like the features for the price. With this system, I get 20 extensions (any extension can receive a fax), you can set up one extension as a dedicated fax number (faxes are received via email and/or their desktop software), you can fax directly from your desktop with their desktop software, you can set up a direct line and receive calls over the computer, click-to-call buttons (like the one on the right side of my blog), etc. A lot of features for about $50/month.
I've used CallWave in the past, which is very inexpensive, but the features are rather limited. You get one virtual extension and a virtual fax delivered via email or desktop software. I don't think you can fax out from this service, but I've never really tried.
I was also invited to beta test GrandCentral, which was recently purchased by Google. I've got a number set up, but I haven't really had a chance to try it out.
I really like that I can hand out one number and the virtual system will forward calls to all my phones. With RingCentral, you can even have it ring multiple phones at the same time.
Anyone else out there using similar systems? Having any success with them?