Learn vim or emacs. You can program just about any language with either of those. You won't have the crutches provided by some IDEs, but you really don't need them. You can become a better programmer when you rely on your brain more and intellisense less.

Both vim and emacs have a big lot of features and extensions (or plugins) to facilitate coding (including things related to APIs, that you won't need to "memorize blindly"). Many programming languages (even academic ones) have their emacs mode and/or vim plugins or extensions. And many Linux distributions are packaging these.

Eclipse can handle most of the languages you mention:

• C# with the Improve C# plugin (Dead since 2004),
• ActionScript with the Eclipse IDE for ActionScript3 and AS Development Tools plugins
• CSS with the CSS Editor,
• Haskell with the EclipseFP plugin
• Scala with Scala IDE,
• And Java is Eclipse's core target language

Regardless Eclipse is not a valid option, as most of the aforementioned plugins are in various alpha states and running an Eclipse instance with all of them would enormously stress your hardware.

Even with great hardware, the plugins will rarely offer everything a specialize IDE will offer. Also I don't see any valid reason to running multiple IDEs, how many platforms can you code for simultaneously? If what you need is mostly to read code written in different languages or write fairly small programs you could choose Notepad++, that supports syntax highlighting on an impressive number of languages.

But to actually program in the languages you mention, I'd recommend to use different IDEs:

• Eclipse for Java, Haskell and Scala,
• Visual Studio for everything related to .Net,
• Adobe Flash Builder for ActionScript,
• Aptana Studio for HTML, CSS, JavaScript, PHP, and Ruby

Adobe Flash Builder and Aptana Studio are based on Eclipse so if you go with those you will at least get similar IDEs.

I personally find Eclipse is the best multi-language IDE.... apart from the excellent Java support Java there is pretty mature development support for most other major languages available as plug-ins.

There's a long list here of languages which I'm sure has been extended since that article was written.

Also it's not just the languages that are important: you also need to consider:

• Build / dependency management tools like Ant and Maven
• Source code control (git, CVS, SVN etc.)
• Resource file management (XML config, images etc.)
• Project management tools (backup, documentation, task management etc.)

Taking all these into account, it's definitely a win to stick to a single primary IDE.

The Jetbrains company has a suite of IDEs with the same ergonomic principles and same keyboard shortcuts and so on. They cover all the languages you're talking about. But most of them are not free though.

So my suggestion is:

• Resharper for C# and .Net: http://www.jetbrains.com/resharper/
• IntelliJ IDEA for the rest: http://www.jetbrains.com/idea/

For languages that have them you're best off using best of class IDEs - they're not really that different. See other answers for suggestions.

For the ones that don't then I suggest you use a more heavyweight text editor - such as emacs (which also doubles up as a file system browser, mail and news reader, gaming platform, birthday greetings generator, etc, etc, etc). These will often have decent plugins/settings/modes that get you most of what a simpler IDE will give you.

Beware that IDE usage is a holy-war topic.

Use the best tool for the job. If an IDE is good at language X and not at language Y, then only use it for X and get something else for Y. Compared to pretty much any computer game you might pick off the shelf, and IDE is nothing your hard drive and system RAM can't handle. I'll add that when tasting a new language an IDE can make it easier to start playing around, don't be afraid to get the one that fits the job. Android Java is a good example of that- if you just want a taste there's nothing easier than Eclipse/Android to get your feet wet.