Without a shadow of a doubt the most useful bit of “shiny kit” I own was my Letherman PS4 squirt. I have a very old original full-size Leatherman that I have used for years, but the PS4 was just the right size to hang on my keys without being noticed. Sadly, last summer it died, but Leatherman have a very good warranty service, so I was not too concerned. However, they no longer make the PS4, and they replaced it with the next size up. While I like this new multy-tool it is too far too large for key ring carry.
Next, I bought a Gerber vice, just don’t go there.
I friend recommended the SOG PowerPint. Sadly, this is also just too big for key ring carry but is perfect to drop in my pocket. In fact, I think I have carried it just about every day for the last 3 months, both on and off duty. It has a nice selection of tool, and you can grip a screwdriver bit with it for extra reach.
The only downside is the very small blade is a locking blade so would be a problem for UK carry unless you have good reason.
Over all though if your in the market for multy-tool that can be dropped in a pocket or bag without being noticed, but cover all those little job you did not know how you managed before you bought it, I can highly recommend the SOG PowerPint.