Can I use the Ruby plugin for IntelliJ IDEA Community Edition?

Short answer

No, but you can port it to there.

Longer answer

Officially, the Ruby plugin is only available for the (for-sale) IntelliJ IDEA Ultimate Edition. Proofs:

• IntelliJ IDEA comparison of editions, showing that Ruby/JRuby support is available as a free plugin but only for Ultimate Edition.
• JetBrains lists the Ruby plugin in the IntelliJ IDEA Ultimate Edition plugin repo, but not in the Community Edition plugin repo.
• StackOverflow reports that the Ruby plugin cannot be loaded into IntelliJ IDEA Community Edition. The question was answered in 2010-04, but since the error message includes a hint that the plugin requires the Ultimate Edition, in accordance with current docs, I'm pretty sure this is still the case.

 So the official alternatives are to buy IntelliJ IDEA Ultimate Edition and install the free Ruby plugin, or to buy RubyMine. The functionality is roughly the same; for the remaining differences and help for the decision, JetBrains has an article up.

However, the concerned Ruby plugin is free software, licenced with an Apache 2.0 licence. Which means that it's legally possible to make it work on IntelliJ IDEA Community Edition. Maybe with a little loss of functioality as some required API might be missing, but not much: custom language support plugins are completely possible for Community Edition. It's admittedly not the nicest thing to do, as the JetBrains folks have really good quality software and reasonabe prices. It would probably not hurt too much as many professional developers would still rather buy RubyMine from them to have a simpler tool that is specialized on Ruby only. Anyway, it's possible and legal, but I did not find any project on the web doing this. Maybe somebody has the missing link for me (or feels inspired to start this thing …).