We use confluence. The hardest part I found about documentation is writing it. To encourage continued documentation, I have suggested in the past to at least create a page in your tool, prefix it with "DRAFT" and at least do a brain dump or even copy paste from past conversations. At least that way, the knowledge isn't lost. I've found that has time goes by, I eventually go back and start cleaning things up and eventually end up with some pretty clean documentation.
For personal documentation, I follow the same thing but with evernote.
By simply having the data somewhere, it can at least be indexed and searched even if it's ugly.
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