TikZ manual newest version online?

The place to look for the PGF offical released manual would be the CTAN directory: ctan.org/pkg/pgf.

You could also use the web site that Stefan Kottwitz and Paulo Cereda recently put together. Just enter

http://texdoc.net/pkg/<packagename>

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 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.