XGooglers has an entry on the use of MySQL to run their adwords program. Adwords is the source of 99% of Googles Advertising Revenue, so it certainly a business critical application. There are some interesting tidbits in this posting about how Google tried a commercial, closed-source database but it was never able to perform as well as MySQL. A few quotes from the same:

* "AdWords was built using the MySQL database, which is open-source and therefore available for free. It is by now also nearly as full-featured as the best commercial databases, but back in 2000 this was not the case."
* "Many managers seem to have this idea that it is invariably true that you get what you pay for, and that therefore nothing that is available for free can possibly be any good. Using MySQL was acceptable as an expedient to get things up and running quickly and with a minimal of capital outlay, but now that things were settling down it was time to recognize that this was really, fundamentally, a mistake, and it should be fixed sooner rather than later."
* "We finally decided to go with a commercial database (I won't say which one) over the objections of a number of engineers, including myself. To ease the transition it was decided to convert AdWords over to the new system first, and to do the main ads system later."
* "To make a long story short, it was an unmitigated disaster. The new system was slower than molasses in February. Some heroic optimization efforts eventually produced acceptable performance, but it was never as good as the old MySQL-based system had been."
* "I have heard through the Grapevine that they eventually went back to MySQL. (Since then, MySQL has added many of the features that had been missing at the time.)"

Source: 1Prashanth RaiTag(s):Open+Source, MySQL