Prefix Index Link is a Userscript which adds a link to Special:PrefixIndex to the top toolbar. The source can be found here. It is based off of a similar userscript that Omni Flames created for Pending Changes Reviewer, which can be found here.
Installation
editTo install the script, simply copy the following into your common.js page.
importScript('User:Gestrid/PrefixIndexLink.js'); // Linkback: [[User:Gestrid/PrefixIndexLink.js]]
Note: You will need to manually bypass your cache after installation. Visit Wikipedia:Bypass your cache to see how to do this.