Almost every software/tool that developers use on their job these days are extensible in some ways. If we can use little bit of creativity, imagination & off-course coding skills, we might make most out of these softwares. We can extend & make them do what we want them to do.
Here are some very easy, basic, simple & yet useful extensions/plugins examples with explanation that you can quickly learn. You can take these as starter & develop further to suite your development needs.
Have a go through & If you make something interesting, do share back !
Develop simple useful eclipse plugin to Scan java source | Good example for beginners
Custom chrome browser extension code | Add modify request headers | Good example for beginners
Custom maven plugin development | Add/modify files in target, access dependencies through plugin