Useful Tools

For Microsoft Windows

WinMerge: File and directory comparison and merging. It's open source, and it's free.

Fiddler: Allows looking into HTTP requests (e.g. headers, content, etc.). Extremely useful when debugging and developing AJAX applications.

Lutz Reoder's .NET Reflector: Allows looking into .NET assemblies and see how they work.

Microsoft Visual Studio: As 13th of December 2007, I am working with the 2005 version, but will probably move to the 2008 version pretty soon.

Aptana: I use this IDE mostly for developing JavaScript, as it provides Intellisense-like functionality for it. Visual Studio 2005 does not provide Intellisense functionality for Javascript yet (this feature should be included in VS 2008).

GIMP: Very similar to Photoshop. It does all I need, it's open source, and it's free.

Web-based

Google Analytics: Great tool for monitoring your website(s).

DynDns.org: They provide a range of DNS-related services. I use them as primary DNS provider for my domain names. They have DNS servers located at different geographical locations, so DNS propagation times are relatively low, which is good.

NetCraft: Great for finding out what web server is running behind a certain domain name (e.g. Apache? IIS?).