This is a bookmarklet you can use to create a card in Trello from ...
GitHub issues and commits
The selected text from an arbitrary URL
... more? I'm happy to take pull requests that add support for other websites!
The first time you run it on a site, it will walk you through a simple setup:
Input your API Key (which you can get at https://trello.com/1/appKey/generate)
Authorize the site to interact with Trello
Select the list that you'd like the bookmarklet to add cards too
You'll only need to go through those steps once per domain; from then on, you should be able to send your issues/cases/whatever directly to Trello in a single click.
View Bookmarklet Code:
Bookmarklet Source Code: