Do you feel like playing Civilization, but only have a web browser handy?  Maybe you should visit http://www.freeciv.net (http://www.freeciv.net/) because with the magic of HTML5 (no flash!) enterprising developers have managed to implement a fully playable version of the game just for your browser.
This is an amazing open source project, and I think a great example of bigger and better things to come in the next few years with HTML5.

I would like to clarify the difference between this and (for example) the browser games offered by EA.  Those games (Tiger Woods, Battlefield Heroes, etc) actually use the browser to download and install a native client on your system, and then that client runs inside your browser.  Like Flash (a very common client, but still a client nonetheless), these things are only available on the platforms for which they were written.  The EA games ONLY work on Windows.  Flash WON'T work on iPhone or iPad.
HTML5, conversely, is not a plug in client.  It's a standard.  Any browser that says it supports HTML5 will work with freeciv.net, regardless of what OS you happen to be using (Windows, Linux, OS X, Android, iPhone, etc).
For more examples of cool stuff written in HTML5, search for "chrome experiments" or "mozilla game on", and remember, there's no special code being installed to your system.  It's all being done by the browser itself.  (I'm aware of the "loading" message in freeciv, but that's just copying graphics and world data to a safely sandboxed storage area on your disk.)

Some more cool HTML5 effects in your browser.  Here's colour cycling (nothing in the picture moves, but it is animated just by rotating the colour palette)

http://www.effectgames.com/demos/canvascycle/