Digital distribution sales of software in the PC market nearly surpassed that of retail in 2009, according to a recent NPD study. Citing poll estimates, the firm found that 21.3 million “full-game downloads” were purchased digitally, in contrast to sales of 23.5 million at brick-and-mortar stores.

Not surprisingly, Steam headlined NPD’s list of “frontline” digital retailers — those that specialize in selling retail equivalents in digital form. Trailing behind Valve’s magnum opus was IGN’s Direct2Drive, followed by Blizzard.com, EA.com, and WorldofWarcraft.com. On the other side of the pond, Bigfishgames topped the list of “casual” digital outlets focusing on smaller, low-budget productions.

All in all, digital sales took 48 percent of market activity in the digital sector last year, leaving the remaining 52 percent to retail.

Interestingly, frontline digital retailers gained market share over their casual equivalent in the second half of 2009, a shift that NPD says stems from increased popularity of free social network and mobile gaming.

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Read more: NPD: PC Digital Download Sales Overtake Retail In First Half of 2010,  Digital Distribution Sales Surge Despite Retail Decline,  Steam Eclipses 30 Million Accounts Mark,  Valve: Digital PC Gaming Market Is Just Swell, Retail Dying,  NPD November: 360 Sales Catapult, Black Ops Dominates