Awesome cheatsheets for popular programming languages, frameworks etc.
lecoupa.github.io
lecoupa.github.io
Rosetta Stone was a good idea, but it kind of fell into a algorithms zoo rather than actual help in cross-language fluency. I guess stackoverflow fills that void from a more scattershot direction, but when it comes to things like basic "library" operations, java has the JDK, Guava, Apache Commons, and a half-dozen other libraries for most tasks.