TiddlyHost is really a freemium service – you can pay for a premium account if you like. This is primarily a way to give back to the community, though; the only important thing you get by paying is revision history (which is nice, but you can always back up your wiki manually instead).
TiddlyWiki itself is completely free, and using the free version of TiddlyHost will not limit what you can do with your TiddlyWiki itself in any way.