I recently chose a development environment for a spare time project: I am re-working some of my old algorithms and miscelanious code (in several different programming languages) for extracting semantic information from plain text after reading through the excellent book The Text Mining Handbook: Advanced Approaches in Analyzing Unstructured Data. I have been working on information extraction for about 20 years (very much part time), and although most of the material in this book was familiar I found the book to be an excellent reference and a good summary for the state of the art in information extraction techniques. I have blogged before about the excellent Reuters/ClearForest system - the authors were principles at ClearForest.
I chose for this project the combination of Gambit-C Scheme, Emacs, and a few customizations of the Gambit-C Emacs code. For "mostly thinking" projects like my information extraction library, I like simplicity: a simple clean programming language and an environment that provides good editing and debugging support but otherwise stays out of my way. Professionally, I do a lot of work with Common Lisp (either Franz + ELI + Emacs, or SBCL + Slime + Emacs) but since I am basically just experimenting with algorithms I felt like using something light weight. I thought about using Ruby (with either the excellent NetBeans support or TextMate) but I like the ease with which Gambit-C Scheme can be used to build native applications or libraries (compiles to intermediate C) and I will probably want to share my information extraction program (perhaps a free and commercial version) but not release the source code. The performance of compiled Gambit-C code is also very good.