The best AutoDialer and hosted Voice Broadcast system on the web is AsteriskDialer.com. Why choose AsteriskDialer.com? (The Asterisk AutoDialer)

High Volume Dialing.
Each account is capable of over 1,000,000 dials per 8-hour day - more capacity than any other hosted solution.

Pricing at 3.5 cents per minute.
We're the lowest priced web-based voip autodialer / voip predictive dialer service on the market.

Our Virtual Call Center Solution.
Complete with Agent-Popup, sub 1-second connect time, and No hardware or software requirements. Our Virtual Call Center solution is a great way minimize cost, and get up and running practically overnight.

Other AutoDialers:
Indosoft Dialer - A canadian based company that, amongst other things, offers an asterisk-based dialing solution.
Vicidial - Open Source dialing software.
GnuDialer - A basic predictive dialer for contact centers. (GNU GPL)
SineApps - Acompetitor of ours with a myriad of features. (Unhosted system).

Other Resources
Voip-Info.org - A great resource for asterisk-based dialers.