A few key chain sized tools come to mind
1. LEDs - photon, princton tec pulsar
2. Swisstech utilikey / microtechician
3. SAK classic
4. Leatheman squirt
5. Leatherman micra
6. Bison capsules
7. Survival Key Chain
8. Streamlight keymate
9. Not to mention numerous knives which can fit on the key ring (SOG/sypderco/gerber etc)

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Trusbx