Rogers Centre - Toronto's SkyDome, aka "the concrete convertible" - was the first large size stadium to have a retractable roof.  This idea was perfect for the Toronto climate which can be a bit dodgy early or late in the Blue Jays baseball season.  The "lid" is typically kept shut to keep the inside warm and dry, and it is opened up before baseball or football games when the weather is nice outside.  Indeed, most spectators have a great view of the CN Tower which literally towers over the stadium when the roof is open.  One of the benefits for fans is that there is never a rained out baseball game.  The stadium has been officially renamed Rogers Centre (after its current corporate sponsor), but to Torontonians, it will always be SkyDome no matter what the big sponsor tries to do.

There are tours of Rogers Centre three times daily, or you can see the dome in action by watching a  baseball game or football game there.

Please note if you are looking for SkyDome in Toronto, you can only find signs for "Rogers Centre"  

More details can be find at the Wikipedia article on Rogers Centre.