A visualization and experimentation tool for small quantum circuits. Includes common single-qubit gates, but not many multi-qubit gates (e.g. swap and fourier transform). Supports controlled and time-varying operations. Not much support for customization yet.
For example, here is a screenshot of a quantum teleportation circuit. The qubit in A1, which is entangled with B2, is teleported to B1:
The bottom left thing is a representation of the highlighted operation as a matrix. Next to it is a representation of the state after the highlighted operation (each cell is one of the classical states, and the contents represent the amplitude). The bottom right thing is a representation of the final state. Within the circuit, the gates showing percentages are "peek" gates. They are showing the probability of the wire being on (if it was measured at that point), both given that the controls are satisfied and in-addition-to the controls being satisfied.
(Up to date as of Dec 6. Basically just all of the javascript concatenated together.)