I went to this Aquarium in late June of this year (2011). It is not exactly free. It is located in the heart of the Dubai Mall, the world's largest mall. Right there, across from many stores is the world's largest aquarium window, a full 3 stories tall and maybe 60-80 feet wide. Behind this glass is the Aquarium's main feature, an Ocean tank filled with sharks, rays, and many other large ocean fish.
But this is just to whet the appetite. For a fare of about $15, visitors can go through a glass tube through the middle of this huge ocean tank. They then go upstairs to a long trek passing many aquatic exhibits on both sides. Species I remember included Amazon fish, archerfish, rare otters, nutrias, smaller sharks, jellyfish, and penguins -- and much more. For an extra fee, you can ride in a glass-bottomed boat over the top of the ocean tank, and they give you food to feed fish near the top.