Terms of use: Free
File format: URL encoder
Website: http://email.about.com/library/misc/blmailto_encoder.htm
Screenshots:

List of mailto setting:
1. Most basic mailto link
mailto:example%40huntfree.com demo
2. mailto link with email subject
mailto:example%40huntfree.com?subject=example%20subject demo
3. mailto link with email subject, cc, bcc and body message
mailto:example%40huntfree.com?subject=example%20subject&cc=example1%40huntfree.com&bcc=example2%40huntfree.com&body=example%20content demo
Simply fill up the form, above tool will automatically generate complicated link code.