Serendipity interoperates with many external tools and websites. Even if the tool doesn't include Serendipity settings, you can usually make it work pretty easily.
I want to use w.bloggar to create entries. 
I want to use the Performancing for Firefox extension to create entries. 
I want to use Windows Live Writer to create entries. 
I want to use my Flickr images in my Serendipity media library. 
I want to make my plogger gallery look like part of my Serendipity blog. 
I want to add entries to my blog by e-mail. 
I want put entries or headlines on a different web page.