I've always been a fan of ⌘-Shift-4 for taking quick and easy screenshots on my Mac, even with such level-up options as Jing, Skitch, GrabUp and LittleSnapper readily to hand. What I wasn't a fan of, however, was the proliferation of "Picture 1," "Picture 2,"... up to "Picture N" screenshot files on my desktop. Not only was it tricky to figure out which Picture file was which (a task made easier with Quick Look), it was also reasonably likely that at some point I'd overwrite a Picture 1 someplace that I wanted to keep with a brand-new Picture 1 I just snapped and copied off my desktop.
It's certainly not one of the marquee features of Mac OS X 10.6, but there's one little Snow Leopard tweak that's already won my heart: no more Picture 1s! Screenshots to file created via the system key commands (⌘-Shift-3 or -4, add the Control key to capture to the Clipboard instead of to a file) now get a name that's quite a bit more descriptive, including the date and time of the capture. No more file name collisions, no more wondering which Picture is the one you took of that cryptic error message in the middle of the night. Viva Snow!