The beta for Tidalis, a unique co-op puzzler from Arcen Games, launched a few weeks back - but upon initial launch it was missing several cooperative features. Arcen has announced that the latest version includes multi-well co-op play as well as several other improvements to the game.
...new in this version are more art/music background themes, more special blocks, the first few usable items, updates to the level editor, added animation techniques and general polish, more block skins including even better colorblind support, and dozens of more minor features.
The official Tidalis info page has the full list of features in Beta .407 as well as the downloader for Windows or Mac.
Along with these updates comes two new videos showcasing the co-op mode and the game's adventure/story mode.