In another thread someone mentioned this relay:
which seems to work well with OpenSprinkler. Also, I’ve recently bought the following relay:
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:
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.