There are a lot of editors you can use to edit source code but i recently tried Code Browser http://code-browser.sourceforge.net/
You can make folders (and sub folders), a kind of sections, to put the code in eg. a folder for declarations, forms, functions etc.
Instead of one long source file you can divide it so it looks nice and easier to maintain because you only see the relevant part of code.
Because all is done with comment markers in the source you can still edit the file in the normal fnxbasic editor.
Also you can add tools like the fnxbasic compiler so compile it from the editor.
Take a look at the elastic tabs to make a nice layout.
Of course fnxbasic editor has also nice things like the component tree and function list but sometimes i like to use another editor instead of the default one :)