Exxon Valdez spilled an estimated minimum 10.8 million US gallons (40.9 million litres, or 250,000 barrels) of crude oil.
Independent scientists analyzing the slick set the estimate at 25,000 barrels a day, and once BP released the underwater video, they calculated flow rates as high as 80,000 barrels a day.
From a comment in that thread :
at 80k barrels a day, this very well could become the most massive spill in history - EASILY. That's 3,360,000 gallons a day for 90-120 days (assuming the other wells they're drilling do what they think they will - which is another assumption they're making). That's 403,200,000 gallons, dwarfing the Persian gulf war spills in the early 1990s.
A couple of things with this estimate approaching almost half a Billion gallons of oil.