It depends on which path you like to follow.
Struts is popular framework that fit nicely once you've got the JSP/Servlets done. Struts 2.0 the latest, but struts 1 is more in widespread use (mostly maintenance stuff)
Spring is a pretty comprehensive and popular framework these days, personally I'd say its a bit difficult to pick up if you not actually practically using it! But if were to back a technology that will have good demand in the
Java space - it would be spring. I can see it growing!
Flex, GWT & there loads of more frameworks that have a nice niche market!
If you looking for advice the best would be to look around and see what's most needed around you so that you could get more opportunity to work on some of it.
Are you working in some area of Java or are you studying? A bit of back ground on where you are and what you like to do, might help folks give you some specific advice.