You can program other web services and run them on RPi, as long as they use a different port than what OpenSprinkler firmware uses (the default is 8080 unless if you have changed it). For example, some users use the same RPi to host the weather service and use port 80 for that. You can pretty much use any programming language of your choice to write a web service, and run it on RPi. Python is pretty popular, and there are tons of other choices too, like nodejs.