I know that you asked for Open Source tools, but if others are going to propose Notepad and Microsoft products, let me mention Notecase Pro.
It comes in flavors for windows, Linux, and Mac. There are constant updates. There is a user community developing plugins. You can write in different fonts, with font colors and background colors. You can embed screenshots.
It is a hierarchical note manager. The price is reasonable, you can get a single user (any number of computers) perpetual license, or pay more and get a multiuser license.
It is not open source though.