I have a forum that sends users an email whenever someone replies to a topic that they posted. However, given how some of my users don't check email that often, and I was wondering if there was another way of notifying them.
You can send them sms (which is costly) and/or if user possesses an advanced mobile device you can send push notifications which are almost free but requires developing a phone app.
Also you could use twitter, rss and instant messaging bots.
How about being inspired by the StackExchange network way of notifying their users when replies and comments have been made to their posts? Or even by Facebook? With their notification function both popping up in the lower left corner of the screen and with the red circle in the notification part in the top bar?
There's a lot of possibilities, but it really depends on how you'd like to notify them.
If you need help on how to implement such a feature, StackOverflow is the place you should ask, when you face a specific problem.