Here's what made me smile just this morning.
For work, I went to check the earnings release for a company we're looking at, which they've posted on their website in pdf. So I click on the link, the pdf downloads, I click on the downloaded link, and the Q3'10 results pop open in Evince Document Viewer. And that's when it hit me - I didn't even think about going to find a viewer, didn't have to install a viewer. Having installed Ubuntu, the OS included this open source software as part of a minimum package the developers decided the average user should have.
I didn't have to think about it. It just works.