A Guide For Designing E-mails: Emailology
Emailology is a website which aims to guide designers and developers with the best practices of e-mail design.
The project includes an e-mail boilerplate, that is very well-commented and includes hacks for cross-client compatibility, for starting quickly.
A knowledgebase lists the tips and tricks categorized for each e-mail client. It is the right place for finding answers to client-specific issues.
There is a standards guide which presents the universally-supported HTML and CSS tags (it is possible to get client-specific data).
And, a character conversion feature allows copy-pasting your own code for converting the special characters into HTML entities.