A bit mindblowing: put the browser itself in the cloud: Mighty [Hacker News discussion].
This free plugin lets you send an email from within an ExpressionEngine template without the user submitting a form. It works in both EE v1.x and v2.x.
It’s probably most likely to be useful when placed within an embedded template that is loaded using ajax by some user action in the parent template.
Michael Rog has written a similar plugin: Email-from-Template.