Part of the challenge with HTML5 is understanding the range of different technologies and finding good samples. The following are some of the sites I have found most useful.
IE TestDrive
A good set of demos using touch, appcache, IndexDB, etc. Some of these only work with IE10. Be sure to click the "More Demos" link at the bottom for a longer list of Demos in a nicely organized list form.
Chrome Experiments
http://www.chromeexperiments.com/ Chrome browser-oriented sumbitted sites with a heavy emphasis on display technologies (WebGL & Canvas)
Adobe Expressive Web
http://beta.theexpressiveweb.com/ Adobe provides a dozen HTML5 & CSS3 samples. I seem to end up playing the "Breakout" style Canvas demo every time I visit the site.
Mozilla Demo Studio
About 100 varied HTML5-related submitted web sites. If you click the "Browse By Technology" button there are other samples for File API, IndexedDB, etc.
Introducing HTML5 samples
http://html5demos.com/ Specific Tech examples related to the "Introducing HTML5" book by Bruce Lawson & Remy Sharp
HTML5 Gallery
http://html5gallery.com/ HTML5 Gallery focuses on "real" sites - sites that were not specifically intended to showcase a particular HTML5 feature. The actual use of HTML5 tech can vary from link to link, but it is useful to see real-world uses.
FaceBook Developers HTML5 Showcase
A good list of high profile HTML5 applications, games and demos (including the Financial Times, GMail, Kindle web reader, and Pirates Love Daisies).
HTML5 Studio
Another Google site - currently 14 samples of concepts like slideshows, Geolocation, and WebGL using HTML5.