Well, see the other thread for a discussion of water solenoid options.
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1229955902
I'd like to keep price below $15 each. I want the DC at 12V each or less, so I am able to simplify the power supply issues as much as possible. It needs to be fast enough for bit-banging visual patterns in the water flow. It needs to be small enough I can rack them together without too much extra tubing work. Of course, the pulse-on, pulse-off types are attractive from a power usage standpoint, but I think I can support pulse or hold types just with changes to the software.
There is a difference between sinking 400mA on one pin, and sinking 400mA on every pin of a device. Many will just pop due to heat dissipation if you try to do that. And my current experimental solenoids are 600mA hold types, anyway.