SJCL is easy to use: simply run
sjcl.encrypt("password", "data") to encrypt data, or
sjcl.decrypt("password", "encrypted-data") to decrypt it. For users with more complex security requirements, there is a much more powerful API, described in the documentation and illustrated in this demo page.
SJCL is small but powerful. The minified version of the library is under 6.4KB compressed, and yet it posts impressive speed results. (TODO: put up a benchmarks page.)
... read the whole story at bitwiseshiftleft.github.io.