Aloha editor
ASP.net MVC HTML5 Toolkit
Bango HTML5 Application Analytics SDK
Buzz: Javascript HTML5 audio library
CanvasLoader Creator
Cottage is a simple graphics and sound engine implemented on top of HTML 5's canvas and audio elements. It aims to be an easy to use library for Web-based game development.
DHTMLX Touch: JavaScript Mobile Framework for HTML5 Web Apps
DotM.Html5 is a library for ASP.NET in which html 5 controls are available for use
Google webfonts
Free HTML 5 templates
HTML 5 Boilerplate
HTML5 Music player
HTML5Pattern: source of regularly-used input patterns
JayData: Enterprise data management with JavaScript in HTML5 web applications and cross-platform hybrid mobile applications
Manifested: prepare your site to run offline using HTML5
Modernizer: library for detecting aviailable features
Mojolicious is "Duct Tape For The HTML5 Web". It is powerful with no dependencies out of the box with RESTful routes, plugins, Perl-ish templates, session management, signed cookies, a testing framework, a static file server, I18N, first class Unicode support, and much more.
OrgChart is a JS library which draws org (organization) chart (tree) for your tree data
PhoneGap is an HTML5 app platform that allows you to author native applications with web technologies and get access to APIs and app stores
Sencha architect
WebSocket
When can I use...

The LobsterPot HTML5 PivotViewer is a cross browser, cross platform version of the Silverlight PivotViewer control produced by Microsoft. Created by LobsterPot Solutions the control has been written as jQuery plug-in and utilises features available in HTML5.
Wedana (Web Electronic Design Automation Native Applications) is a JavaScript framework and end-user applications that allow you create, edit, and view electronic schematics using nothing but a Web browser. All produced data is based on the gEDA suite's formats, so the can be easily exchanged between each software.

HTML Basix
HTML Center
HTML con clase
HTML Crash course
HTMLDog
HTMLform: build HTML foms online
HTMLGoodies
HTML Hulp (NL)
HTMLite: HTML, Beyond HTML, CSS, Javascript, XHTML, PHP, ...
HTML Made really easy (J. Marshall)
HTML Power tools
The HTML Station
HTML Source
HTML Station
HTML.com
HWG - The HTML Writers Guild
Internet.com
Joe Barta - Professional Web design and HTML Tutorials (PageTutor)
Jukka Korpela: web authoring and surfing
LinxExplorer - HTML link validation tool
Lunascape: hybrid engine browser (Trident+Gecko+WebKit) to check different browsing engines at once
Mike Markel's Web Design Tutorial
Math in HTML (and CSS)
Metatags: zoekmachines en HTML
Microsoft HTML and DHTML Reference
Mozilla Developer Network
Openweb: pour les standards du web (FR)
Pagetutor (Joe Barta)
Position is Everything: browser bugs exposed
Quirksmode: browser compatibility
Safalra's website
SelfHTML (DE)
Sitebuilder
Sitebuilder
SiteExperts
Snapfiles: the place to be for software
Timeforweb
Topics