+1 on Flickr being the best acquisition and product Yahoo! had.
I still have my account and old photos there. And because I licensed most of them as CC, a couple of them landed on Wikipedia because of that - felt nice.
I had everything set as CC until I noticed a photo of my very pregnant wife was getting many more views then anything else and I found it cited in a paper on training AI. That was somehow less endearing then someone getting a good use out of my images (which also happened at least once with one of my images)
When I was doing more graphics-rich presentations, the CC photo resource on Flickr was really useful. (In case someone asks, I usually wasn't being paid directly for giving presentations so I convinced myself I could feel comfortable using CC content in general even with strings like non-commercial attached.)