I often like to link to the corresponding CTAN page if I mention packages in my posts. I always wished there would be a button which does this for me. I now hacked some Greasemonkey user script together which adds one to the button bar on top of the input field. On request I added a second button for the package manuals on http://texdoc.net/.
Simply type the package name, mark it with the mouse and press the
DOC buttons on top. Many thanks goes to Paulo Cereda for designing the icons.
For Firefox this needs Greasemonkey to be installed. There are also alternatives for other browsers, but I can't say if they work.
Note that this does not work in cases were the edit window is not displayed from the start, i.e. when you answer your own question or provide a second answer. Also it might not work if you reload a page. In this case reload it again by selecting the URL in the browser location bar and hit enter.
Also the buttons do not update the preview window. You need to type some text before it is updated.