Secure, sandboxable user exposed programming language / environment?
I use Lua for this, but it's directed at a Lua capable community. So my answer would be who are your users?
If your users are internal, like my case, and proficient with Python use Python. However if this is something for the world wide web, I'd probably choose javascript, because its the lingua franca, (every developer knows it, and its easy to pickup). As for an Engine... well V8 would be nice, but its not 100% thread safe, in that you can't run several engine within the same process in a lock free manner, as you can with SpiderMonkey. So You might want to use that. Also since javascript is sandboxed by default you won't have to worry about implementing much on your side.
Beyond offering an API for my website, I'd like to offer users the ability to write simple scripts that would run on my servers . The scripts would have access to objects owned by the user and be able to manipulate, modify, and otherwise process their data.
I'd like to be able to limit resources taken by these scripts at a fine level (eg. max execution time should be 100ms). I'd also like to ensure a secure sandbox such that each user will have access to only a limited set of data and resources, and be prevented from accessing disk, other people's data, etc.
Generally the scripts will be very simple (eg. create the sum or average of the values that match certain criteria), and they'll often be used in templates (eg. fill in the value of this cell or html element with the average or sum).
Ideally I'd like to use a sandboxed subset of a well know, commonly available programming language so it's easy for users to pick up. The backend is written in Python, so a Python based language could have benefits, but I'm open to other languages and technologies. Javascript is also attractive due to its simple nature and common availability.
The languages should support creation of DSLs and libraries. 
The target audience is a general user base for a web based application, not necessarily very technical. In other words, it's not targeted at a base with particular knowledge of any particular programming language. My expectation is a subset of users will create scripts that will be used by the larger majority. 
Any ideas or recommendations for the language and technology? Any examples of others trying this and the successes and failures they encountered?