Certainly there has to be some clever people out there putting things on the frame where the bottle cage normally goes. Sure there are a lot of bottle openers (which is ironic, where does your bottle go?) but has anyone seen other, weird, handy, clever accessories mounted to those two or four little screws?

I'll start with the easiest. beer bottle opener

I leave the bottle cage and drop in a Jif peanut butter jar with my tools, some tyvek for tire booting and a spare tube in there. Fits well.

I put a burrito in mine once.

I've got a cage mount on the under side of the down tube. I used a spare bottle as tool caddy, but the tube wouldn't fit and so I just combined everything in the seat wedge. But I just may have to follow your example.

Back on topic, I remember someone saying they've been known to stash a rain jacket in the cage.

I just mounted a splash guard cut out of a 2L soda bottle using the two cage bolts on the underside of the downtube of my Bike Friday. Not nearly as good as a full front fender, but it lets me still fold/pack the bike in its suitcase without having to remove the guard. The concave side of the guard faces the front tire so the spray tends to be directed inward and down toward the road instead of hitting the convex downtube and then out to the side and onto my feet.

And I agree with some of the comments above about using a spare cage for holding tools and other small items. A tennis ball container fits well and holds quite a bit.

+1 on a plastic peanut butter jar, filled with "stuff". stuff a sock in it to keep everything from rattling around, put between your hand and a greasy chain, and wipe off your hands after a repair.

Wow, thanks for the ideas, especially using a bottle for tools. Could also put small lights and backup batteries in the bottle?

There are also things sold that look like water bottles but either hinge open lengthwise or screw open in the middle for storing all kinds of stuff. Also there are bike light batteries designed to go into a bottle cage. And the AirZound air reservoir bottle goes into a bottle cage. There are also several tool pouches designed to roll up to the right size to fit inside a bottle cage (or under the saddle)

My pump came with a support that screws in the water bottle cage holes.

This makes me think of the possibility of terrorist carrying a b*mb on the bike

hmm... before 9/11 those stickers used to say THIS BIKE IS A PIPE BOMB

Seems like the best sticker should be "THIS BIKE USED TO BE A PIPE BOMB"

I'm guessing it would be much smarter to just go ahead and use a car or a motorcycle

my first trip into NYC after 9/11, by car, and before going through the holland tunnel all vehicles had to be "inspected" by guys in uniforms with guns. the "inspection" was basically a mirror and flashlight being waved under the car. some cars were being "searched" by visual inspection of the trunk. i guess they were looking for some cartoon-looking "bomb" strapped to the bottom of a vehicle, and forgetting that all motor vehicles on the road were equipped with a GAS TANK that would have made a decent explosive device with very little modification.

as always with "security theater", people who are neither trained nor equipped for bomb detection or bomb disposal are the ones looking for bombs.

you can use your water bottle for storing things (other than water).
good place for a few tools, money, keys, power bar, etc.

the water bottle also acts as a security device. a thief wont steal a water bottle because they think there's just water in it. a seat bag on the other hand, would be a target for them.

Friend of mine who lives out in the country has had problems with dogs. One of his water bottle holders has a big honker "crowd control" pepper spray canister he got off of some law enforcement officer supply website.
Said it works great, but you just have to be careful not to let it blow back in your face.

Don in Austin

I guess for the fixie crowd, Pedro's Trixie might have some appeal:
https://www.pedros.com/trixie.html
One tool that combines:

• 15mm box end wrench.
• Lock ring hook.
• 8,9,10mm box end wrenches.
• 5mm hex.
• Of course, a bottle opener.

Would be nice to somehow hold the tire repair kit--any idea?

Shimano Pro Storage bottles are available in 500 & 750ml sizes and were intended for tools and tire repair.

I have the pouch that holds the Abus folding link lock
on the water bottle cage mounting bolts, of my Bike Friday.

they have a second, low behind the seatpost cage mount,
I use that site to place the bottle and under the cage I use
some mudguard struts wrapped around the bottle cage
to provide the 2 places to mount the dynamo powered
taillight, that would normally be fitted to a rear rack..

i use a similar light on my other bike that does
have a [Tubus] rear rack..

Mid 80's in Houston, I had this junk yard dog coming out and chasing me every morning and the owner knew it and ignored it. I sprayed water in it's face every time to keep it off. on a 30 degree morning, I took a tea pitcher of ice water and dumped it on his head. he never looked at me again.

I had a mail carrier, dog repellant bottle on the bike but squirting a water bottle in thier face always backs them off.

Read the thread. A plastic peanut butter jar holds a spare tube, tire levers, a multitool, patch kit and some booting material. I also posted a photo of a roll-up kit that fits in there nicely.

I've been talking to Otterbox about making a nifty little waterproof tool and toy box to mount on a quick-release bracket secured by the water bottle cage bolts, and a "wedge" for under the seat. They appear to be too busy trying to kiss up to the iPhone 5 crowd to make anything useful, though.