You can use a pen and paper if you like to write everything by hand or you can download or write your own software. There are also some online journals however I personally just download something someone else has written... there are lots of journal software both free and paid, I just happen to like free better.
Here are my picks for free software journals...
- RedNotebook | How can I describe it in a way that most people will understand... Awesome, just about sums it up! For screen shots, video, and downloads hop on over to their site http://rednotebook.sourceforge.net/
- iDialyDiary | Another good program go here to find out more. http://www.splinterware.com/products/idailydiary.htm
Well I'm not going to waste my time on this most of the ones that claim to be free only have 14-30 days of free usage before they want you to buy it, and a few of the others want you to pay to unlock more features.