I've been using Windows and Visual Studio for a long time, and although I really liked it, I find it too heavy for my current tasks. I'm looking for a lighter editor that meets my needs.

Here's what I'm looking for:

1) A lightweight editor compatible with Mac OS X (optional, but preferred. I have limited storage and would rather not use up additional gigabytes.)

2) Support for highlighting HTML, C#, and JavaScript (since I work on ASP.NET projects, these are the main languages I need).

3) The ability to auto-format code (I used to rely on pressing CTRL+K CTRL+D every few minutes to format my code, which I found convenient).

4) Auto-completion functionality (optional, but it would be nice to have).

In addition to these requirements, a customizable interface and easy integration with version control systems would be great additions to the editor.

Based on your requirements, I recommend considering the following lightweight code editors:

Visual Studio Code (VS Code)

Compatibility with Mac OS X: Yes
Support for HTML, C#, JavaScript: Yes, through extensions.
Auto-formatting: Yes, by default on save or by pressing Shift + Alt + F.
Auto-completion: Support through IntelliSense.
Customizable interface: Yes, you can customize almost everything.
Integration with VCS: Git support is built-in.
Sublime Text

Compatibility with Mac OS X: Yes
Support for HTML, C#, JavaScript: Yes, built-in.
Auto-formatting: Available through extensions like AStyleFormatter or JsPrettier.
Auto-completion: Yes, but not as powerful as VS Code out of the box. Can be improved with plugins.
Customizable interface: Highly customizable.
Integration with VCS: Via plugins like GitGutter.
Atom

Compatibility with Mac OS X: Yes
Support for HTML, C#, JavaScript: Yes, built-in.
Auto-formatting: Available through packages like atom-beautify or prettier-atom.
Auto-completion: Supported through the autocomplete-plus package, which is included by default
Customizable interface: Yes, you can customize both the UI and the syntax themes.
Integration with VCS: Git and GitHub integration is built-in.
Among these, Visual Studio Code seems like it might be the best fit for your needs, as it's lightweight yet powerful, deeply customizable, and has robust support for the languages you're using. Microsoft created both Visual Studio and Visual Studio Code, you'll find a lot of familiarity in the latter if you've been using the former. However, all three editors are very good, so it might be worth giving each a try to see which one you prefer.


Here are a couple more simpler and lightweight editors:

Brackets

Compatibility with Mac OS X: Yes
Support for HTML, C#, JavaScript: Yes
Auto-formatting: Available through extensions like Beautify.
Auto-completion: Yes
Customizable interface: Bracket themes can easily be changed.
Integration with VCS: Git integration available through brackets-git extension.
This software is primarily focused on web development, with live preview options for HTML and CSS files.

BBEdit

Compatibility with Mac OS X: Yes
Support for HTML, C#, JavaScript: Yes
Auto-formatting: No built-in, but it supports scriptable automation.
Auto-completion: Yes
Customizable interface: Interface elements are customizable.
Integration with VCS: Basic integration with Git using command line tools.
This tool is much simpler than the likes of VSCode or Atom, but it's also extremely lightweight and known for its reliability. It might not have all the features you're looking for, but if you want something that's truly unobtrusive, it might be a good choice.

Remember, selecting an editor depends heavily on your personal preferences and workflow. Consider testing these options to see which one suits your style best.

In case none of these options are up to the mark, there are other options available too like WebStorm (Not very lightweight but very comprehensive for JavaScript), TextMate, and others. However, the ones listed here are some of the best and most widely used options.


Additional editors that are light and could potentially meet your needs:

Coda (Nova is the successor now)

Compatibility with Mac OS X: Yes
Support for HTML, C#, JavaScript: Yes
Auto-formatting: Available
Auto-completion: Yes
Customizable interface: Decently customizable interface.
Integration with VCS: Git support is built right into the sidebar.
Coda, now Nova, is a fast, clean, and powerful text editor. It is specifically made for Macs and is primarily aimed at web developers with support for remote file management.

TextMate

Compatibility with Mac OS X: Yes
Support for HTML, C#, JavaScript: Yes
Auto-formatting: Yes
Auto-completion: Yes
Customizable interface: There are many bundles/themes available.
Integration with VCS: Supports Git through a simple Git Bundle.
TextMate bridges UNIX underpinnings and GUI by cherry-picking the best of both worlds to the benefit of expert scripters and novice users alike. It's a versatile plain text editor with a unique and innovative feature set.

Komodo Edit

Compatibility with Mac OS X: Yes
Support for HTML, C#, JavaScript: Yes. Also supports Python, Perl, Ruby, and Tcl, plus JavaScript, CSS, HTML, and XML.
Auto-formatting: Yes
Auto-completion: Yes
Customizable interface: Multi-window editing, multi-select, a toolbox with shell command integration, and more.
Integration with VCS: Git integration is available.
Komodo Edit is a great editor if you're looking for something powerful, yet simple.

Whichever editor you choose to use will mostly depend on your individual needs and preferences. All of these editors are capable and competent, and many people swear by each one. It's definitely worth doing some extra research and perhaps trying each one out to see which one feels the most natural and intuitive for you to use.

WebStorm is the best web editor, there's no comparison with VisualStudio - it's like heaven and earth. Once you've used WebStorm, you won't want to go back to VS. For Mac users, there's also Coda which many people like, but in my opinion, WebStorm is superior. If you need more than just front-end development and want support for other languages, go for IntelliJ IDEA right away. It has everything that WebStorm offers, plus support for other languages.

When it comes to asp.net, it's important to note that it's a vendor lock-in that requires Windows. While there is MonoDevelop available, it's not as visually appealing and can be frustrating to work with (though it might be the only option for some). If you value freedom of choice in operating systems like Mac or Linux, then it's best to steer clear of asp.net.

Introducing the newly available TextMate 2, a beloved and established text editor.

With the convenient shortcut ⌥⌘[ (Text = Indent Selection), you can easily apply simple formatting.

Additionally, TextMate 2 offers a wide range of language-specific bundles, including formatters for different programming languages.

If the built-in options do not meet your needs, you can even write your own custom autoformatter using the console.

The possibilities for customization and productivity are endless with TextMate 2.

I recommend trying Visual Studio Code (VS Code). It's highly customizable, supports HTML, C#, and JavaScript with excellent syntax highlighting, and has a plethora of extensions available. You can easily set up auto-formatting with the Prettier extension, which will give you that CTRL+K CTRL+D convenience. Plus, the IntelliSense feature provides robust auto-completion. The interface is clean, and it integrates smoothly with Git, making version control a breeze.

If you want something truly minimal, try Atom. It's open-source and customizable, but some users find it slow compared to VS Code. It offers decent support for your languages, but the auto-completion can be less reliable.