Says Director of Research & Analysis, Benjamin Schlichterv (via GIW):
Steam's growth has been remarkable during the year. Over 180 titles were estimated reaching over $1 million USD in revenue for the year, painting a very healthy market for developers and publishers, with more room for growth in the future.
The top title on Steam was Call of Duty: Black Ops, which sold an estimated 1.6 million copies in just two months, bringing in $98 million revenue.
Here is the rest of the list (title - estimated revenue):
Call of Duty: Black Ops (Activision) - $98.2 Million USD
Call of Duty: Modern Warfare 2 (Activision) - $39.4 Million USD
Left 4 Dead 2 (Valve) - $36.0 Million USD
Battlefield: Bad Company 2 (Electronic Arts) - $25.4 Million USD
Sid Meier's Civilization V (2K Games) - $21.9 Million USD
Portal (Valve) - $20.0 Million USD
Fallout: New Vegas (Bethesda Softworks) - $17.0 Million USD
Metro 2033 (THQ) - $13.4 Million USD
Mafia II (2K Games) - $11.9 Million USD
Warhammer 40,000: Dawn of War II: Chaos Rising (THQ) - $10.8 Million USD