This entry is a Bike Hacks Classic.
When you commute or run around town you often lock up and run in and out of places. You still face the threat of that minor mechanical problem or flat tire so tools are a necessity. A variety of small bags for tools are designed to fit under your seat or possibly connect to your frame, but they are not easy to remove and if left on your bike can be appealing to others.
When doing some online searches I ran across the Cage Rocket. It fits in any standard water bottle cage and you can store essential tools you might need on the road. The great thing is that when you leave your bike locked up, the bottle slips right out and you can easily take it with you. Get more info on the Cage Rocket specs from their site.
You can see that it is not huge, but I am able to put in a multi-tool, tire levers, and a zip lock bag full of miscellaneous items like zip ties, small bungee cords, band aids, and safety pins. To keep stuff from rattling around I also stuff a small rag into the compartment.
Their customer service is also great. When my bottle ejected when I bunny hopped a pot hole and the lid broke off they sent me a new lid to replace the broken one. They also offer a water proof version called the Spod.
I thought I ran across a post somewhere on the Interweb a while back where someone made their own cage rocket of sorts using a plastic water bottle. I didn't bookmark it and but if you have done such a thing or have found such a hack feel free to pass it on to us.