Best site for coding interview practice?

I'm spending most of my off hours with any number of dev stack technologies - RoR, Angular, Meteor, Python, R, D3, etc - basically anything that looks moderately interesting. I've developed a razor-thin knowledge layer of each. Not enough to impress a hiring manager, but enough to keep going.

I've invested in all the right books (O'Reilly & Pragmatic probably love me), have spent time on a few e-learning sites, and read cloned GitHub repositories late into the night.

There's little chance of using these tools in my day job. (Dilbert has NOTHING on that place.) Building toy apps that gather dust in a forgotten corner of GitHub is fine - but they don't tell me whether I'm any good.

I've heard of companies using InterviewZen to screen applicants. Can anyone recommend a good site for self-testing?