As you point out, Project Euler and other simple puzzles are good for picking up new languages. Of course, Euler actually has a broad set of tough problems, but many people prefer solving the first tenner using one-liners.

It's slightly vintage now, but there is nothing better for getting actual algorithmic problem solving ability per time invested than USACO at http://train.usaco.org/usacogate. It was developed by the US high school competitive programming coaches. If you can plow your way through the first couple of sections, even the toughest interview questions will seem easy.

Serious.