Well, not really an IDE, but a web application in which you can write code in any language, compile it and execute it.
I’m talking about Ideone, where I landed last week while searching for a place where I could write so Ada83 code to see if I remember how it is done. The site supports many languages, from the mainstream (C, C++, Java, Ruby, etc.) to the legacy (Ada, Cobol) to the more obscure (Whitespace, Unlambda).
Excellent place to test ideas and short code snippets. Thanks guys!