Is there a definite place where TikZ manual can be found online?
I am only finding version 2.00, while I keep loosing the link to 2.10.
Does the official site host the manual somewhere, or is it only available in the zip? Any help appreciated.
If you need the absolute most recent version of the manual (even for unstable builds of PGF/TikZ) you can look here: http://pgf.sourceforge.net/ There you can find the manual for the current unstable version of TikZ.
The place to look for the PGF offical released manual would be the CTAN directory: ctan.org/pkg/pgf.
into your browser's address field and you'll automatically be taken to the right place. So in your case, go to http://texdoc.net/pkg/tikz or http://texdoc.net/pkg/pgf, they both lead to the manual, version 2.10.
Joseph Wright wrote about this service on our blog: TeX documentation online: texdoc.net
If you want a CVS version of pgf tikz, I put on my site altermundus the complete archive with the documentation and the changelog (it's the version of 24/01:2012) I will put the last one in a few days.
The source for this package is on Github (Aug 16, 2019). The manual is created as part of the continuous integration process and uploaded to: https://pgf-tikz.github.io/pgf/pgfmanual.pdf
As of 2019, the documentation (manual) version 3.1.1 can be found in pdf form at: CTAN-math-washington-edu: TikZ & PGF. It's north of 1200 pages, so I suppose this is the complete manual.