If you use sticky notes (either physical or digital) to remind yourself of things to do, then you might want to try out Gumnotes.
Gumnotes is a free tool that lets you attach notes to any file or website. If you are working on a file and want to be reminded of something, you could attach a note to the file using Gumnotes. The next time you open the file, a notification will popup, clicking on which will open the attached note. Pretty convenient.
You can use Gumnotes with Word, Excel, Images or just about any other document. You can use Gumnotes to annotate websites too and it does not matter which browser you use.
You can also set a timer for a note to remind yourself at a particular date and time.
Gumnotes integrates with Simplenote and so you can view and edit your notes from anywhere including your iPhone or Android mobile.