Let’s define some base parameters: web application development with the JVM as targeted platform (choose your poison: Java, Scala, …). Here’s my list and some hints on why I think them more worth than one look:

Wicket

Nothings more thought-thru for developing web applications.

iBATIS

There’s a big hype about all those ORMs like Hibernate or EclipseLink. But when it comes to performance, flexibility and easyness, I think iBATIS the better approach. It’s very mature and with the upcoming next major release (a beta will be availabe in Q1 this year hopefully) it’ll gain some Java 5 toys to play with.

Quartz

When in need of scheduling tasks one shouldn’t have to bother on implementing anything else than the tasks. There are few libraries as mature and reliable as well as easy to develop with as this one.

Compass

Built on top of Lucene this library simplifys integrating search into you applications immensely!

TestNG

I think unit tests are overrated when you look at how they’re used in many cases – often it’s just for numbers and not quality testing. Every programmer should and needs to dive a bit deeper into this matter!

I left out other great libraries cause they don’t appear as often in my projects but are nonetheless very valuable. Among these are (for the records): Guice, FreeMarker, JFreeChart, iText, SpringSecurity, Joda Time, LOGBack.

And did I say that all of you should use Scala?!

Best regards, — Jan.