These are some of my favorite software utilities for development and life in general. Everything listed here is free.

IDEs

PyCharm Community Edition (JetBrains) - A feature-rich IDE and debugger for Python built on the IntelliJ platform, with the free community edition having arrived in 2013.

Windows

Notepad2 (Florian Balmer) - This is a notepad replacement that adds some nice features, like syntax highlighting, the ability to open large files, and many more. It's not a whole friggin' IDE, though, so it retains the lightness of the original notepad. If you get and run the installer download, it will automatically replace the Windows notepad.

Cmder - A replacement for the Windows Command Prompt. If you split your time between Windows and Linux, this will add joy to your life. It includes the common *nix utilities, like ls, cat, tail, grep, curl, git, etc. by virtue of including msysgit. It also supports ANSI color codes. There are (still, I think, as of early 2015) occasional glitches when it will become briefly unresponsive, and sometimes performance problems when running in a VM, but still totally recommended.

Regular Expressions

regex tester (regular expressions 101) - A great browser-based regular expression tool with specific support for PHP, Javascript and Python. There's also a community-created collection of regular expressions.

Regular Expression Designer (Rad Software) - This is a downloadable utility to test .NET regular expressions. There are many such utilities out there, but unlike some, this one you can actually figure out how to use it by just looking at it. There's also a handy reference in the right pane.

Decompilers

ILSpy (SharpDevelop) - Most people weren't too happy with .NET Reflector's free version poison pill. This is a free alternative.

dotPeek (JetBrains) - Another .NET Reflector alternative, in "early access". I haven't used this one yet, but stuff from JetBrains tends to be solid.

JD-GUI (Emmanuel Dupuy) - A Java decompiler that is pretty simple and works well.

Misc. Python Tools

virtualenvwrapper (Doug Hellmann) - virtualenv is great, virtualenvwrapper makes it even better by simplifying the process of creating and switching between virtual environments.

virtualenvwrapper-win (David Marble) - Windows port of virtualenvwrapper.