I use tiddlywiki, which is opensource. It's been recommended by many HN comments in the past. It works pretty well. You can store a copy on dropbox, and use and

I use tiddlywiki, which is opensource. It's been recommended by many HN comments in the past. It works pretty well. You can store a copy on dropbox, and use android/ios apps to sync it. The web interface on a computer is really great. The mobile apps are not as great, though (clunky around syncing it).