Upnote is a toy clone of Notational Velocity a note taking program. Both Notational Velocity or nvALT, a fork of the original NV, run only on OS X. Upnote uses Qt C++ (c++11) to hopefully be cross platform.
Upnote development can be found at https://github.com/digikata/Upnote
Upnote lacks many features that nv and nvAlt, but you can use it with a directory of text files. Right now it can load and search a set of text files in a test directory. It's still very early it's development, so I wouldn't trust any critical files to Upnote at this point.
Upnote is open source. See the LICENSE file that applies to the source code of this software.
You'll need a Qt dev environment, qmake and optionally Qt creator to build Upnote. Qt 5 is recommended.