Are there any source control systems that require another user to validate the source code "before" it can be checked-in?

I want to know as this is one technique to make sure that code quality is high.

Update: There has been talk of "Branches" in the answers, and while I feel branches have there place I think that branchs are something different as when a developer's code is ready to go into the main branch it "should" be checked. Most often though I see that when this happens a lead developer or whoever is responsible for the merge into the main branch/stream just puts the code into the main branch as long as it "compiles" and does no more checks than that. I want the idea of two people putting their names to the code at an early stage so that it introduces some responsibility, and also because the code is cheaper to fix early on and is also fresh in the developers mind.