Quantum computer can be simulated by classical computer with exponential slowdown.

Here are some quantum computer simulators: http://www.quantiki.org/wiki/List_of_QC_simulators

No mention of LIQUi|> (pronounced liquid) in this list. http://research.microsoft.com/en-us/projects/liquid/ MSR claims to have the most advanced / fastest quantum computer simulator.

As I understand, LIQUi|> is not publicly available. Is it?

I believe you are correct, so you cannot just hobby hack on it, but it you have a serious purpose I think the team will listen to you.

In any event "List of QC simulators" appears to be just that, a list, with no stipulation that it contains only open source software, etc.

another one that seems missing is this one http://www.quantumplayground.net which runs in a browser and has tutorials on how to write programs.

EDIT: spelling

It's just an exponential overhead. The state of a quantum computer is a superposition over all bitstrings, i.e. a vector of size 2^n for n qubits, and you have to implement quantum operators by multiplying with 2^n x 2^n unitary matrices (you better do that implicitly whenever possible or it will be very slow).

After taking a lecture on quantum computing, it was actually a weekend project to implement a basic quantum simulator myself, very useful to confirm that I actually understood the model as well as I thought.

In fact, you don't even have to understand quantum mechanics to create a quantum simulator. Mostly just linear algebra, and how states evolve through time, given some Hamiltonian.

This sounds really interesting! Do you have by any chance a link to some useful resources ?