Last year, we reviewed Arcen Games' Tidalis, and we were quite impressed. It was an editor's choice 5 point review, which we don't give out too terribly often. One of the most impressive features was the easy drop in/drop out co-op for local play. Tidalis can be played with either mouse or keyboard, and if someone walks by interested in playing, they can just grab whichever of the two the first player isn't using. It's a perfectly seamless system, but Arcen wasn't content to rest on their laurels.
The latest patch for Tidalis adds in another keyboard control scheme, bringing the upper limit for local co-op play on one computer to three. Yup, that's 50% more co-op, for the low, low price of free. Other changes in the patch include improvements in performance and minor bug fixes. If you are a puzzle fan, and enjoy co-op, check out Tidalis; you'll not be disappointed!