Gamasutra has US PC sales data from NDP showing digital distribution seems to be on the verge of eclipsing physical sales of games, with NPD estimating digital distribution represented 48% of unit sales (21.3 million units, compared with 23.5 million physical units) for PC last year, accounting for 36% of dollar sales. They note this shows the NPD is attempting to capture some previously ignored data: "Retail sales of PC games declined 23 percent in the U.S. to $538 million in 2009, NPD reported in January this year. As online sales from downloadable content, full game downloads, subscriptions, microtransactions and mobile continue to increase, NPD is looking at ways to measure the impact of those emerging models on a regular basis." They also offer some analysis of this and the first article also shows the top five digital distributors based on share of unit percentage:

1. Steampowered.com
2. Direct2drive.com
3. Blizzard.com
4. EA.com
5. Worldofwarcraft.com