At my current job I've been working on making web apps with Java, Tapestry, Hibernate, MSSQL, and Tomcat.
I've got an idea for a little web game I'd like to write. I'd like to know what the SO community would use for something like this.
Should I stick to what I know? I was thinking it would be very beneficial for me to learn different languages and frameworks. Any suggestions?
If its a personal project then take this as an opportunity to learn something new.
Spring Framework using Spring MVC 3.0
I recommend learning something new because even if your idea is a failure, at least you learned some new technology out of it.
If your thinking about an online game like a RPG or card game I might be selfish and suggest you have a look at Project Darkstar and let us know how you got on. I've been trying to get started with it and cannot get it up running in my environment.
I can suggest Grails as it is a modern and easy to use RAD Web-Framework, has a small learning curve and is proven to be scaleable.
It builds upon Spring MVC, Hibernate and other sophisticated frameworks and merges them with the dynamic nature of Groovy (dynamic language for the JVM) into a full-stack webframework. It follows modern principles like CoC (Convention over Configuration) and DRY (Don't repeat yourself). You can deploy the applications straightforward as .war file in tomcat or any other servlet container or applications server.
The answer isn't language dependent, see this question and responses:
http://stackoverflow.com/questions/349318/what-other-frameworks-should-aspnet-programmer-consult
My recommendation would be to see what you could do with a combination of Google App Engine, Django, and JavaFX.
In doing so, you'd learn some new technologies (App Engine) and languages (Python for Django, JavaFX) while taking advantage of what you already know about web apps and Java.
PHP and MySQL is an easy place to start for web apps. Your ISP will support it and there are tons of tutorials on the web.
Once you have got the hang of it, take a look at MVC architecture. It's a way of structuring web apps.
Personally I'd then recommend looking at CodeIgniter.
However a game is a different matter, maybe try Flash?
If you want to improve your skills push what you already know. If you want to learn new languages and ways of doing things, then look at alternative technologies and stacks.
It all depends on what your goals are...