I found this Reef Fanatic solenoid at Aquacave
to be the right combination of convenience and price. Careful, some solenoids (Burket.. Burkit?) will require some moderately technical assembly of the power cord. The Reef Fanatic one comes fully assembled and even has a LED indicator to let you know when it's on and co2 is flowing.
I also like
, but it took returning my first package to get ones that worked. I think quality control is lacking, plenty of others have the same story of DOA units... but now that I have working ones, I love 'em. They're small & unobtrusive, fit in a power strip, are set digitally for accuracy, and keep their time when the power goes out.