Web-based to-do manager Todoist already integrates with Gmail forwards and opens from Launchy, but a "Todoist Anywhere" bookmarklet makes the service easy to integrate with Gmail labels or individual messages, as detailed in this video.
As noted in the video, you can sign up directly from the pop-open bookmarklet, and clicking the bookmarklet while an email message or search/tag results page gives Todoist a little special context for your tasks and sub-tasks. Got another tool to integrate Todoist, or another task manager, with your Gmail? We'd love to hear about it in the comments.
Todoist Anywhere [Todoist]