Many applications have the built-in capability to send emails. For example, you can send a document from Microsoft Word, a PDF from Adobe Acrobat, or photos from Windows Explorer. The problem is that these don’t work with web-based email.
Affixa allows you to easily attach files to new messages in Gmail, Google Apps, Zimbra, Office 365, Outlook Web App and Yahoo! Mail.
Oh, and it handles “mailto”s too, so when you click on an email address, you can choose which mail account to use.