One of them is ALEX. ALEX Personal Knowledge Base is a very simple tool to organize your knowledge, notes, ideas and other information. ALEX PKB is FREE to all type of users to use anywhere. The source code is available under GNU General Public License. ALEX PKB is platform independent. You can run it seamlessly on Windows32/64, Linux32/64, OS X, Solaris, AIX and etc. you can download ALEX for free here.