In the last two years I found the following software development tools to be quite interesting, for their quality and usefulness, already good two years ago, but steadily improving:
* Mercurial: an open-source multi-platform distributed revision control system.
* Redmine: an open-source multi-platform web-based issue tracker, wiki, and project tracking system.
* Scons: an open-source multi-platform Python-scripted build utility.
* Sparx Enterprise Architect: a proprietary open-format UML tool. (Still waiting a comparable open-source alternative).
* Mono+GTK#: an open-source multi-platform .NET implementation, comprising C# and VB.NET compilers, a .NET virtual machine, an internationalization-oriented GUI library (GTK#), and an integrated development environment (MonoDevelop).