I've been using Evernote for 5 years now. Google Keep doesn't make a good notes system other than for casual to-dos, writing a quick number, etc. It takes a leaf from actual sticky notes, like Todd said, but yeah, you can be much more productive with the actual thing.
That's why I keep a set in my bag. For me, they're useful as footnotes/writing calculations, having them stick out as a bookmark, and for writing a quick note to place on another piece of paper.
And I can still take them all and put them in Evernote.
Springpad is good too, but it's more like a bookmarking service than anything else.