I didn't have room to do an arduino with a bunch of shield boards so the arduino board I'm using is a custom design, it is electrically equivalent to the standard duemilanove boards except I added a real time clock and a 512kb EEPROM and shrunk the board size by about 20%. I haven't used the EEPROM yet but I hope to use it to log my temperature, ATO consumption etc and I could also store moon cycle/season/tidal data in there so my lighting and powerhead cycles can change with the moon and seasons.
The second board is my LED drivers, none of the off the shelf drivers I could find had the dimming performance I was looking for since I wanted to dim my main array down far enough to use as moonlights. I built some based on National's LM3404 and they work great, the blues never turn off all the way they just slowly dim down to a minimum setting at night.