Zim is great too. In terms of functionality, it does a lot of the things Obsidian does, but it uses its own wiki syntax and it's not pretty by any means. Obsidian has been much more effective at building community and growing the ecosystem of plugins.

Edit: And one pretty annoying thing - it's built on old-school technology that doesn't even support setting the editor width. That's quite a problem in the era of widescreen monitors.