Excellent site in English and Italian giving a history of the Basilica as well as panoramic views, e-postcards and information about up-coming events.
It is free to enter the Basilica - although there are often long queues in busy periods. To avoid the queue you can book online - it costs 1 Euro and is available between 1st April and 31st October. The same link enables you to book guided tours.
There is a charge to enter the Treasury, a charge to see the Pala d'Oro and a charge and climb up to the balcony overlooking Saint Mark's Square. Similar links in: Churches