jsqubits is a JavaScript library for quantum computation simulation.
Try the online jsqubits runner. Alternatively, you can download and use the JavaScript library from the jsqubits github repository, or installing it from npm. It is fully self contained — no third party libraries are required. You can use it to implement quantum algorithms using JavaScript like this:
jsqubits('|01>')
.hadamard(jsqubits.ALL)
.cnot(1, 0)
.hadamard(jsqubits.ALL)
.measure(1)
.result
Why JavaScript? Simply because it can run in almost all web browsers without having to install anything. You can even use the online jsqubits runner on a smart phone!
Being JavaScript, you will not get great performance, and will probably find it fails to handle more than one or two dozen qubits, but it is the sort of tool I wish was around when I first started learning about quantum computing.
If you are new to quantum programming, then it is highly recommended that you try reading John Watrous' Quantum Information and Computation Lecture Notes. You may also wish to try reading the partially finished Introduction to Quantum Programming using jsqubits or my article An Interactive Introduction To Quantum Computing