Is there a plugin for gedit to enable me to have the API for a particular language open in a sidebar?
I'm currently working in Java, and it would be useful to have a tree type document with the details of the API keywords.
I am not sure if that can be achieved in gedit, but there are several alternatives for this case, such as Bluefish which imho is the most lightweight and will give you the reference information in autocompletion popup as shown in the Edit Preferences.
Give it a try, it isn't that different from or more resource-demanding than gedit.
A php file highlighted and html reference information in a popup.
An html file highlighted and html reference information in a popup.
Edit/Preferences Language support config.
Supported languages, which include but are not limited to Javascript
You may consider contributing to the community by writing a Plugin for the job that you want to achieve. Here is a good starting point from Gnome team.
These aren't plugins for gedit, but you might like them: The program devhelp
and the website http://www.gotapi.com/html