1. Notepad++: It's a lightweight open source text editor in place of editplus/textpad/wordpad etc. I always wanna have it in my machine, for quick editing configuration files etc. for java.
2. Execute Query: Simple & Lightweight SQL editor for java develper, I loved using it.
3. Tortoisesvn: It's an Open source Subversion client to work with your source code repository.
4. Open Office: It's an open source office products such as word, math, power point etc.
5. Eclipse: Universal Open source IDE, no doubt no java developer can live without this.