Web browsers, and some native applications, support directly sending an item to MarsEdit for posting. When you send an item, MarsEdit opens a new window, ready for editing, with pertinent context from the sending app inserted into the new post.
For example, after enabling MarsEdit's Safari extension, click the MarsEdit rocket icon in Safari's toolbar. Safari sends the selected page's info directly to MarsEdit.
MarsEdit formats the incoming item with a template, stored in the Quick Posts section of the Preferences window.
The Insert Placeholder popup menu allows you to insert template items representing information about the incoming item: title, URL, etc.