Kall8 is more than just an 800 number - it's got voicemail/fax capabilities, detailed call logs, emailed messages and faxes, custom routing that can be setup based on different hours or on-the-fly, and a slew of other features.
If all you want is a typical 800 number you can get one from your phone company for about $0.03/minute and as long as you make a minimum number of calls there is no setup or monthly fees.
For small businesses, however, Kall8 offers a great service. I use it for my web development company and like how I can route the calls anywhere (like if I go out of town I can route them to my cell).
However, our e-commerce business has a traditional 800 number as we get calls on it ALL DAY and our bill is several hundred dollars a month (just the toll-free charges).