If you're a Linux user and not a fan of faux 3D beveling and flashy gradients, then I strongly recommend the Numix Project: http://numixproject.org/
In particular, I'm using their Gtk+ theme, their icons, a Firefox theme, and one of their wallpapers.
I kept Firefox open for the screenshot because I notice a striking similarity between Numix's aesthetic and Google's style new to last year, which both have squarical flat shapes and colours everywhere. I like it.
Gtk+ ThemeDownload it from the Deviant Art page linked above, and unpack it into ~/.local/share/themes/ (you can find that directory by opening the file browser, and typing in the path directly or choosing to see hidden files and folders and navigating there). You should have a directory ~/.local/share/themes/Numix (and for ~/.local/share/themes/Numix - Gtk3.4) now. Then open GNOME Tweak Tool (gnome-tweak-tool, you may need to install it), go to the Themes section (you might need User Themes extension installed but I doubt it), and set GTK+ theme and Current Theme to Numix.
Icon ThemeGo to the github page linked above. In the bottom right, there should be a "download zip" button. Download it, and unzip it. This should give you a folder "numix-icon-theme". Go in there and copy the Numix folder into ~/.local/share/icons/. Then go to GNOME Tweak Tool again and set the Icon them. Next, be delighted when opening Activities view of GNOME Shell.
FirefoxSadly, Firefox does have great support for this yet. If set the Gtk theme to Numix, many of Firefox's widgets won't theme at all and will look like GNOME 2.0. However, that's not so bad. You can at least spice up the tab bar by downloading the Numixy theme by visiting the above link. Google Chrome apparently has an official Numix theme provided by the Numix Project.
Things wantedI don't usually put too much effort into theming my environment because a lot of themes grate on me after a while, and because they're often incomplete. They're not actively maintained. Someone creates two hundred icons, but then stops creating new ones for new applications. That's why the Tango project was special, good guidelines and motivated people.
It would be nice to have GNOME Shell itself be themed (I think the Numix developers are Ubuntu users though) and to have better support for Firefox. Anyway, I'll enjoy it while I can. :)