Use languages in which most characters don't matter

This is a generalization of Mama Fun Roll's point about BF. An esolang that ignores most characters is very useful in polyglots. Also useful: an esolang in which a large set of characters are interchangeable. Some examples:

• Whitespace ignores everything that isn't space, tab, or newline.
• Brain-Flak basically ignores everything besides ()[]{}<>. (@ sometimes causes an error when the interpreter tries to parse it as the start of a debug flag.)
• oOo CODE ignores everything except letters. Furthermore, all lowercase letters are interchangeable, as are all uppercase letters.
• Wierd only distinguishes between whitespace and non-whitespace characters.
• In Wordy, some punctuation characters are ignored, and all letters are interchangeable.
• Both Parenthetic and Parenthesis Hell ignore everything except parentheses.