You should read all the agreements before signing up.  Dreamstime keeps your images for six months and say they can change any of their policies at any time so if you submit an image you have agreed to any future changes in the agreement no matter what they are for six months.  Veer is putting your files up on Alamy even if you already have the same images up there and then taking a cut of those sales, therefore charging you to compete against yourself.  Fotolia says they can lower your ranking to the lowest level if they feel like it and you are selling images somewhere else for less money. 

Funny, I interpreted the question as one about the uses for which you would be licencing images...  If so, lots of variety.