The 4x4 grid sizes: there are 200 levels. Obviously these are the easiest....but I've suffered many deaths. At the moment, I've done about 75 levels.
There are 200 more levels in the 5x5 series. Haven't touched them.
In the 6x6 series, also 200, I've done 102. Yes, they're tough.
Then there's the 'filled 4x4' which is very interesting. And hard! See the last image for an example of that.
There should be a leaderboard to see who else is playing. Very, very good game.
Similar games (this list is incomplete):
Movez Pretty much the same as Block Mania but it has 1680 levels. I've solved 592 of them. That's probably enough.
Frozen Ice Cubes What's different: there's a point of 'no return'.
Escapology, reviewed by Nicola Salmoria. He told me about this game ages ago. I played it and enjoyed it, but never blogged about it.
Block Mania Similar to Movez
Move Puzzle - A Funny Strategy Game, Matching Tiles Within Finite Moves
Vubu What's different: the vubu men have different powers. Very sophisticated, very intelligent. Possibly the very best in this genre.
Move: A Brain Shifting Game