Jim Dunlop JDGT121 System 65 Complete Guitar & Bass String Change Tool Kit

It fits in your case or gig bag, alongside your spare set of strings that  you definitely carry in there at all times, right? RIGHT? Perfect.

From Dunlop: This kit includes the following pro-quality tools that you can use to change your strings in no time with no frustration:

Universal Speedwinder
Compact String Cutter
Superlube Gel Pen
Hex Key Multi-Tool
Microfiber Polish Cloth