I'm very fond of the tiny stuff- it's seldom the very best you can carry, there are some compromises for really small size, but comparing it to full-sized equipment is REALLY missing the point. IMHO, really tiny equipment that works adequately is worth a lot more than heavy/bulky equipment that works even better.

In the "tiny" category I'd put:

The aforementioned tiny whistle.

Photon 3 (No, not "2". Water resistant, easy to change batteries, latest model with auto-SOS feature).

BSA Hot Spark (ferrocerium, key sized, just barely large enough to work fine).

USGI P38 Can Opener, doubles as a fine striker for the Hot Spark.

REI "Featherweight" mirror, thinner than a credit card, I cut it down to fit an Altoids-size tin. Hole for signalling. Others (Campmor) carry the same thing, different names.

"Permanent Match" lighter, the plastic version - some don't like 'em. What's not to like? Tiny, waterproof, refillable, multi-fuel, backup ferrocerium and striker if it runs dry. Lights lamps, stoves and fires easier than a lighter.

Mini Bic - if you insist, but it's harder to use for some things, won't work in serious cold, etc. Admirably small and light, though.

"Flapjacks" folding reading glasses - for those who need 'em, a 1 inch by 3 inch oval when folded, it's in the tin.

Wallet-sized Fresnel lens - mostly as a last-ditch backup to above for reading, but might start a fire.

One unusual item that I carry is one of those garden variety needle threaders, the little flat metal tab with a wire loop on it. The aggravation it saves if you have to use your needle and thread, especially in bad conditions (raining, failing light), is well worth the tiny weight and bulk, IMHO.

I usually carry a Leatherman Micra, and I own a few Vic Classics, but I don't really include them in this category. They're small, but IMHO they don't dramatically outperform their weight and bulk. There's no real trick to making a tiny version of something, it's making a tiny version of something that still works well.