In another thread someone mentioned this relay:
http://www.automationdirect.com/adc/Shopping/Catalog/Relays_-z-_Timers/Electro-Mechanical_Relays/Power_Relays,_Open-Style,_40A_(AD-PR40_Series)/AD-PR40-1C-24A
which seems to work well with OpenSprinkler. Also, I’ve recently bought the following relay:
http://www.amazon.com/gp/product/B005T742IA/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
which is what a lot of pump start relays use, and it works with 24VAC input as well.
Beyond these, another option is to use a RF transmitter combined with a remote power socket to control any power line device, as described in this post:
https://opensprinkler.com/opensprinkler-firmware-2-1-1-new-feature-control-remote-power-sockets/
This method is a bit more complex to set up but if you want to control multiple devices it’s a more ideal solution and it’s easy to scale up.