Java

JDK 1.1

'Nuff said. Just do it.

JDK 1.2

'Nuff said. Just do it!

Visual Cafe

Commercial (of course) but it does finally work with JDK1.2. JDK 1.2 is not all that well integrated into the Visual Cafe environment, but it does work. I have the "Professional" version, but the standard is probably much the same for most purposes and only costs a hundred.

Netbeans

A pure-Java debugger. Join their early access program and get the JDK1.2 version ("X2"). It's a little sluggish but seems to work almost as well as Visual Cafe, plus the integration with JDK 1.2 is much better (as it runs in JDK 1.2!). The thing I really like about it is that you don't have to mess around with projects like in Visual Cafe.

Together/J

A pretty awesome UML Java tool. Free trial version doesn't allow saving or exporting, but still well worth a play. Real licenses are expensive.

Argo/UML

Free UML editor written in Java from UC Irvine. Fairly fluid interface, useful for very basic UML diagrams. Needs more work in the editor -- it comes with source so maybe you can contribute to it.