Comment by Surio

I like to store my data locally, so that influences the choice of my note taking software.

I use cintanotes portable myself... http://portableapps.com/apps/office/cintanotes-portable It is not "opensource", but it has been most useful for keeping notes, tagging, and categorising

I also use http://portableapps.com/apps/office/keepnote-portable (http://keepnote.org/) which is FOSS and also http://portableapps.com/apps/office/rednotebook_portable (http://rednotebook.sourceforge.net/) which is also FOSS.

Choose which ever you feel comfortable using.

P.S: I am also a very big fan of the portableapps format and the platform. Big shout out to John T Haller for all his hard work on this!

P.P.S: I also use the org mode at times http://orgmode.org/ :)