Isn't topcoder still the king of this kind of thing. I don't think they particularly market it as being geared at hiring but companies definitely hire from there (I know UBS likes to) and your score does reflect on your ability, at least as far as being able to code competitively.
- https://www.hackerrank.com/ (aka https://www.interviewstreet.com a YC company)
- https://codeeval.com/
- https://coderwall.com/
- https://www.employiq.com/
- https://www.mindsumo.com/ (college student only)
- http://www.codewars.com/ (in beta)