Wolfram Workbench - Mathematica Development Alternatives

There are indeed some open source alternatives, as other posters have suggested, but you will miss the unique facilities of WB to develop state of the art documentation. So if you want to develop some serious work in MMA, for yourself or others, you should seriously consider WB. Having said that, I use WB in a (probably) unconventional way. Within WB you can select which editor you want to use for the various file types. The default being: editing the .m file with the internal WB editor. Well, I instead chose to edit the .nb (package) file using the standard front end (linked to WB), this action will automatically update the .m file and then use all the standard WB facilities to integrate documentation. In this way you have the all the cool front-end editing tools plus all the cool WB documentation and debugging tools at your disposal.

This technique is described in more detail in my answer in Managing formatted usage messages in Wolfram Workbench

There is also a TextMate bundle for Mathematica:

https://github.com/dehowell/mathematica-tmbundle

TextMate is for Mac only, but this bundle work in other editors like Sublime Text (multi platform) (windows)

Also for Atom (multi platform) there is this: https://github.com/Fitzse/language-mathematica

Another alternative is the IntelliJ IDEA IDE with the Mathematica plugin. See this post for details:

Open-source IntelliJ IDEA plugin to support Mathematica development