The bookmarklet is a neat little thing that lets you post something you find online without leaving the page you’re on. Click the bookmarklet to open a post form in a new window. It’ll be pre-filled with things from the page you’re looking at, including source information (one less thing to worry about). Add anything you want to the post, then either publish it immediately, queue it for later, or save it as a draft.
To install it, go here and drag the "Post to Tumblr" button to your bookmarks bar.