Development Software
Brackets - A very easy to use modern opensource texteditor which is very well suited for web design and coding. It has a very visual hands-on approach with a menu assisting you when you need it. It also supports plugins that can help you with indentation.

Aptana - A development suite with git integration, html, css and js assistance. Also features an integrated debugger.

Stylizer - A css tool that speeds up the styling process of your website with minutes.

Online HTML Editor - It is WYSIWYG. For in some cases when you just need to quickly see the changes you did.

Atom - Like Brackets, an open source text editor with html, css, js, and other scripting language highlights. Supports a large number of plugins

Sublime Text - Another text editor like Brackets and Atom but with some cool mass editing features.

Webtools and services

Gfycat - A website that can convert and compress gif images to the gfycat format. It will then display as HTML5, reducing the file size immensively. You can also convert videos to be played in gfycat, which is

Online WYSIWYG HTML Editor - A very useful online what you see is what you get html editor.

Debugtools

WarmRoast - Developed by sk89q and it is a JVM CPU Sampling tool which can help you figure out why your server is lagging.

Server software

nginx - nginx [engine x] is an HTTP and reverse proxy server, a mail proxy server, and a generic TCP proxy server.

WampServer - WampServer is a Windows web development environment. It allows you to create web applications with Apache2, PHP and a MySQL database. Alongside, PhpMyAdmin allows you to manage easily your databases

Monitoring

GoAccess - Analyzes webtraffic log files and presents them to you in a summarized way.

munin - Makes graphs of various system processes: disk space usage, connections, bandwith, ram usage, cpu, disk io errors, etc. It is very customizable.

Other tools

everytimezone.com - A graph that shows all the major timezones in one view.

PortableApps - Tons of applications in a portable version.

Spigot Documentation - The Java documentation for spigot and bukkit currently, be aware that this may not be the same version the server is running on so test before deploy.

Ninite - A site that can bundle one click installers of popular software for you, without the risk of unchecking (or checking) an option that installs crap. Highly recommended!