Make sure they offer telephone backup if the system goes down. A great way to investigate this is to try calling customer service in the heart of a busy trading day. If the system seems to move with the speed of a Florida vote count, that's a fair indicator of poor reliability. By the same token, email a question to the brokerage house and see how long it takes to get a reply. Certain brokers change their prices considerably depending on the size of the trade, so make sure that the bargain basement rate applies for the sorts of trades you'll likely be making.
Some houses offer thousands of mutual funds while others only deal in a handful.
Gomez, a Boston-based research outlet, ranks online brokers according to reliability, customer service, ease of use and other factors. Jeff Wuorio is a correspondent for Money magazine and writes columns for several other national magazines.