Windows/Mac/Linux (Firefox): list.it combines the keyboard-tap availability of Firefox's sidebar with the helpfulness of always-there, synchronized text notes. After installing the extension and setting up your shortcuts for the sidebar, note searching, and the quick-input bar (the defaults are, unfortunately, Mac-specific), you're pretty much set up to start tapping away. If you're frantically browsing and searching for gift ideas or last-minute work research, being able to quickly enter notes that get automatically backed up across Firefox browsers is no small help. If you're feeling generous and non-private, you can opt to let MIT's Haystack research team examine your notes and timing in their studies of productivity, memory, and the like.
list.it is a free download, works wherever Firefox does.