Frequently I find myself opening an R or Julia session to generate some test data, do a quick calculation, or plot something. This is designed to be an in-browser computation language that satisfies that need.
Hosted during development here: http://kevbase.com/old/scicomp
The code is inspired by Mihai Bazon's lisperator, Peter Norvig's lispy, and Marijn Haverbeke's parse-js.