Almost each language has some additional technology or skill or whatever which can work in a pair with it but still be something fresh. For example, Java + Flex. It's a good pair - those who learn Java and want something both useful and new may try Flex. What are "pairs" for the most popular languages(Java, C#, C++, etc.)?

PS: Most people advise learning functional programming as an additional skill but this is very fuzzy. They talk about such abstract things as wide programming perspective and other things, but you can hardly say whether these functional skills will be really needed. Yeah, maybe some basics of it can be useful, but serious learning of LISP seems not perspective.