[Galadriel55]

Short answer: lies. Long answer is... long.

- In the powers and knowledge of all other Valar he had part, but he turned them to evil purposes, and squandered his strength in violence and tyranny. For he coveted Arda and all that was in it, desiring the kingship of Manwe and dominion over the realms of his peers. ~The Sil, Of the Enemies

- He began with the desire of Light, but when he could not posess it for himself alone, he descended through fire and wrath into a great burning, down into Darkness. ~The Sil, Of the Enemies

(I remember a more general quote that Morgoth wished to have everything better than the other Valar, but when he could not reach that by "nice" means he started messing up Arda. I might be making something up here, though, since I can't find it.)

- The Shadow that bred them can only mock, it cannot make: not real new things of its own. ~LotR, The Tower of Cirith Ungol

- Yet the lies that Melkor [...] sowed in the hearts of Elves and Men are a seed that does not die... ~The Sil, War of Wrath


So based on those passages we can say that Morgoth "created" two things: lies and evil. Both are in the realm of philosophy that I will not attempt at this late hour. However, he didn't really create even that, since concepts exist even before anyone "discovers" and uses them... As soon as there is harmony (Ainulindale), there is the possibility for the opposite (Morgoth's banging). The whole thing about opposites existing only when there is an opposite....

I'll leave that for tomorrow. Enjoy the quotes. They give more information than my ramblings.