Guacamole allows you to access to your VNC server using just a browser, an HTML5 + JavaScript (AJAX) viewer for VNC, which makes use of a server-side proxy written in Java. The current version is almost as responsive as native VNC and should work in any browser supporting the HTML5 canvas tag.

Read more about Guacamole and its features in its official site.

Update: As Mariana says in the comments, another web based VNC alternative is ThinVNC, a pure HTML & AJAX Remote Desktop implementation.

Hakim El Hattab, an interactive developer from Sweden, has created some really great HTML 5 experiments using javascript, the canvas element and some physics.

Client-side storage is one of the most interesting features in HTML 5 and it has a lot of uses, specially for browser apps. If you want to start experimenting with local databases, Ben Lister came up with a great tutorial about this feature: Getting Started with HTML5 Local Databases.

HTML5Rocks is a great website that combines an interactive presentation about HTML 5 with tutorials and a very cool playground, where you can play with some of the HTML 5 features while watching the code.

Google, Mozilla and Opera have joined foreces in a new high-quality, open video format that could end the ongoing debate about the video support in HTML 5: the WebM Project.

Henry Jones makes a great selection of useful HTML5 tutorials and cheat sheets for WDL.