The Skewb Ultimate or Ultimate Skewb is a more complex version of the skewb. It's the same shape as a megaminx but has the lovely skewb mechanism going for it. It isn't quite as easy to solve as the skewb but not the hardest thing in the world to deal with.
It's a Meffert's puzzle and can be found on his site. You can also find these on ebay too, possibly for a bit less. It isn't the most common item in the UK, and, as a result, some people over-estimate the value of the puzzle. Check out the retail price from Meffert's before you go crazy in an auction. As ever, be wary of the word, 'rare' when included in an ebay listing. It's a fairly large thing as puzzles go and pretty good quality.