Tesseract is a 3-dimensional block-packing game, inspired of course by that other 2-dimensional block-packing game, and more specifically Frac and Frac4D. Try it out here.
Use the arrow keys to move blocks in the X and Z directions (i.e. along the plane of the floor). Press V to rotate the current piece. Press the spacebar to drop a piece. Alternatively, use the number pad to move pieces in 8 different directions (includes diagonals, with 1, 3, 7, and 9), rotate with 5, and drop with 0.
Frac (and Frac4D) both use a slightly odd projection called oblique, or cabinet, projection. Ideally it should be possible to alter the projection simply by swapping out the projection matrix with an orthogonal projection, which would allow more intuitive rotation controls (mouse-look anyone?). There are some bugs however that have prevented me from making this work perfectly, so it's not supported right now.
Adding the fourth dimension is a higher priority, and it's not clear to me that free rotation would make the game easier to play, although rotation by fixed degrees certainly would. (Frac4D docks the player points for rotating the board, and I find it extremely disorienting to rotate while handling a piece in 4-dimensions, so I think there's some improvement that could be made here, and intend to explore it).
It's written in JavaScript using WebGL, and depends on the quite nice gl-matrix library for matrix math, but nothing else, except a "modern" browser: I tested with Chrome, Firefox and Safari on Mac OS X, and they all work, but you might need to fiddle to enable WebGL depending on your browser and OS. If you find a problem with it, but this site works for you, please let me know!