What is the default font that Mac OS X Lion use in it's apps, more importantly the Finder window?
Here are the default font settings for Mac OS X
as displayed by the Bresink TinkerTool utility. TinkerTool also permits the user to change these defaults, within certain parameters, as explained in the screen shot. I am posting this screen shot here because it provides a nice chart of all the fonts and sizes and how Mac OS X uses them.
The default system font in OS X is Lucida Grande, below is a screenshot that compares a TextEdit window and Finder. Also, this Wikipedia article describes all the fonts included with Mac OS X 10.7 and notes that Lucida Grande is the default main system font.
In response to mockman: Quick Look applies the same styling that you'd see by opening the html file in Safari. Like any browser, Safari has a set of default CSS rules which define how un-styled content is rendered. The typeface you're seeing then, is likely derived from Safari/webkit—and not OS X.
(I'm still new here, so I'm not sure if it's appropriate to respond to questions raised outside of the original question—which has now been answered. Please feel free to remove this post and/or set me straight if need be.)
I know this question is old, but for folks coming from the future (like me): the current macOS system font (since 10.10) is San Francisco.