Accurately referred to as the ‘Russian Versailles’, Peterhof (pronounced Petergof in Russian) is one of Russia's number one tourist attractions. It is particularly spectacular from late-May to mid-October, when the 147 fountains are turned on, but even so it is well worth visiting at any other time of year.
The Great Palace was built between 1709 and 1724 under the directions of Peter the Great, but the tsars and tsarinas that followed each wanted to make their own mark, adding another palace or fountain to the grounds. The focal point of the whole ensemble is the jaw-dropping cascade fountain leading down from the palace and from there the park spreads out to the left and right along the coastline with trick fountains, pyramid fountains, marble sculptures and other impressive stuff.  
Other museums worth visiting within the grounds include the Hermitage Palace, the Marly Palace and Monplaisir, the little house where Peter the Great used to organise parties for his closest (and rowdiest) friends.