WAtomic is a nice Windows remake of Katomic, which itself is a clone of Atomix by Thalion (except boasting many more difficult levels than the Thalion original).My own review of Atomic, another Atomix clone on this site, applies to this newer game equally well, so I'd like to rephrase it here:

If you have never played Atomix before, the idea is simple: move different atoms around the screen to create the required molecule. For example, in the first level you must move 2 Hydrogen atoms and an Oxygen atom and connect them in the right order to create water molecule. The trick is that each atom will not stop moving until they hit a wall, other atoms, or other obstacles. You must also plan all your moves ahead lest all the atoms get stuck in the corners, forcing you to restart. If you enjoy the Thalion original or just devious brainteasers in general, WAtomic will keep you occupied for many hours. The game is also open source, so any programmer who is so inclined can extend the game can do so him/herself. Recommended!