Keywords for Spotlight!
Searching with Spotlight works pretty well with text, but not so much with images. It is possible to solve that in iPhoto by adding metadata that can be used when searching. Not all pictures are in iPhoto however. A great way to do it is to use the Preview Inspector. You can add the inspector to the Preview toolbar or get it with "Command-I". In the Inspector, there is a section specific for Keywords. The keywords will then be used by Spotlight. You can add these to images and PDFs. I think it really improves the overall experience, because with time, I rely more and more on Spotlight and worry much less of keeping all the folders organized. Here is the Preview Inspector: