Main Tower website (English)
 
Frankfurt tourist office page on various skyscrapers (English)

For a good general view of the skyline, try the Main river bridges such as the Eiserner Steg. Also, if you approach the city from the airport on the S-Bahn suburban rail service, stay to the right side of the train. Just before the train approaches Frankfurt main railway station it enters a big curve, from where you'll have a good first glance of the skyline.

The Eiserner Steg (Iron bridge)
The Eiserner Steg is a pedestrian footbrigde that links the area around Römerberg square with the southern, Sachsenhausen, side of the river Main.  It was built in 1868 as a result of an initiative by local people, who had complained to the mayor that the single bridge over the river that had existed until then was not adequate to handle the increasing comings and goings over the Main. 170m long and made of rivetted steel, the bridge is likely to be your route over the river to Museumufer or to the cider bars in Sachsenhausen.