Amstock is American Stock Transfer. They are one of the primary transfer agents for public companies, as well as one of the largest DRIP administrators, and they handle about 125 direct purchase plans.
Their InvestPower.com web site allows you to purchase or sell registered shares in about participating fifty Direct Stock Purchase Plans, letting you access to information about your holdings in those plans all at once.
Amstock and InvestPower are not brokerage firms; they just run the DRIP and Direct Purchase Plans set up by the companies themselves, and consolidate your holdings into one point of reference. Traditionally with DRIPs you have to deal directly with each company, so buying and holding Amstock/InvestPower companies is likely to be a lot easier than it would have been 10 years ago if you held the same companies.
But there are relatively few companies that are available in this particular agent's roster. There are other transfer agents that offer similar types of online access, which is a great convenience for DRIP investors. So you might find that the companies that you're interested in aren't available in Direct Purchase Plans going this route.
The advantage of Sharebuilder is that you have access to lots more companies -- even those that don't have DRIPs or Direct Purchase Plans. If you just used Sharebuilder, you would have a single point of management for all your holdings, which could be quite a timesaver.
Fees and minimums must be examined carefully in order to see what works best for your budget and your planned schedule of investing. If you're willing to take on a bit more work, investing direct with Amstock or a similar agent could be a money saver for you. And there's nothing that precludes you from using both Sharebuilder and Amstock or setting up DRIPs through another transfer agent -- or even trying both and seeing how it works!
Doug