Professor's Cube (5x5x5)
The Professor's Cube is the name Rubik's gave to their 5x5x5 cube. Although it might take you a while to realise it, the bigger cubes can all be solved using pretty much the same methods, an extra twist or turn here or there is all you need to add to what you know. The first time you try to solve it may take a little longer than you expect - but not too long.
As with all the puzzles, the Rubik's version is not really very good quality. The Eastsheen 5x5x5 or the V-Cube 5 are far superior. It takes me twice as long to solve the Rubik's version because it turns so badly and always feels like it's on the verge of falling apart. The cube in the picture is now only used for ornamental purposes - the alternative cubes are so much easier to use.