TheOpenForce.com

Pingdom, an online web site monitoring service, surveyed 7 massive sites to determine what infrastructure software they're running.  The sites included Meebo, YouSendIt, Alexaholic, TechCrunch, FeedBurner, iStockPhoto and Vimeo, all of which are high volume Web 2.0 sites.  As it turns out the LAMP stack is being used in 6 out of 7 of these sites. According to the article, MySQL dominates the database selection among high volume web sites:

With open source ruling the game it shouldn’t come as a surprise that the database of choice for all but one of the sites is MySQL, the ultra-popular Swedish open-source database.

“The features that you get for free on MySQL, with replication, in-memory and fault-tolerant databases (if using MySQL cluster), transaction support, and the wicked performance, cost thousands of dollars with other database engines,” says Joseph Kottke, director of network operations at FeedBurner.

These sentiments are echoed by the other participants in the survey as well.

“We needed something proven, flexible and low-cost,” says Simon Yeo, director of operations at Meebo.

Update:
I should also point out that 5 out of 7 of the sites run the Apache web server and 1 runs Microsoft IIS (and in fact an entire Microsoft stack) and one runs lighttpd (known as "lighty").  Lighty has been growing very rapidly in the last year and is used on lots of Web 2.0 sites. 

• WebProNews: TechCrunch, Others Love Linux, MySQL
• PingDom: What the World's Most Popular Sites Are Running, Survey (PDF)