Here are some free, online alarm options for those of us (all of us) who need help with the bells.
TimeMe offers an alarm clock that allows you to setup multiple alarms with several sound options, including a traditional school bell. Your alarms are saved as long as you don't clear your cache. I was able to setup class times and tardy bells with different alarm sounds for each.
This site only offers a guitar sound for the alarms but you can setup multiple alarms (1440 according to the site.) It also saves your alarms until you clear the cache.
Google's Timer and Stopwatch
This won't help with the bell situation, but if you need something for setting time limits or timing an activity just Google "Timer" or "Stopwatch" and you'll get the Google online version. It's easy to use and you can project it for your students.