Allen Wrenches, Hex Key, Wrenches - just do the math and you can add to a few more tools to your box.
If you have one set or several sets of wrenches... there always seems to be a time when you still don't have the correct size wrench. The alternative is to do the math and add up any combination of wrenches you have to get to the size you need. This will sharpen up your fractions skills if nothing else.
4mm + 4mm = 8mm Allen / Hex Head Bolt
3/16 + 3/16 = 3/8 Allen / Hex Head Bolt
Any combination will work, just be sure to take the time to correctly fit the wrenches square and tight inside the bolt, before applying any pressure. Righty Tighty - Lefty Loosey
This method has worked for me more times than I can remember. I've used 3 to 4 wrenches and found it to be a reliable and effective alternative to get me down the road.