This configuration works pretty well for me, mainly because I've pared it down to 2 actual keys - one house key and one key for a padlock. I could probably remove the padlock key, but I use it enough that it's nice to have it on me. I keep my car key separate, as there are times I go places without my car or otherwise don't need my bulky car key fob.
Instead of a standard split-ring to put everything on, I'm using a stainless steel screw pin anchor shackle, only I've removed the pin and use a hex bolt with a self-sealing nut on the other end. It's a 1/4" shackle, so the pin is way too thick to run keys through. I've used smaller shackles in the past, and the screw pins on a 3/16" shackle will probably fit most full-sized key heads with a little bit of dremeling, if you want to try this and it's important to you to keep the pin.
However, the pins like to back out after a while, so I switched to the bolt + self-sealing nut, which has worked well for me thus far. Some plumber's tape on the pin threads might keep it in place, but I won't use locktite on my key setup. With my current arrangement, you've also got a screw & nut on you at all times, which might come in handy.
I also have a Brewzer on there. Mainly I like having a bottle opener around, but I've also used the screwdriver, box cutter and hex wrench a number of times each. The oxygen tank thing on the other end I have yet to find a use for. But overall it's a good tool, though there are cheaper solutions like the Gerber Shard or the NiteIze DooHicKey that do pretty much the same things. All of these make for average bottle openers, and that was the primary objective for me, so all of the other stuff didn't matter as much (and the DooHicKey actually didn't exist when I bought the Brewzer).
Next, I have an eGear Pico light. People forget that Doug Ritter designed this, and it's a well-made little light that will take a ton of abuse. I have tried AAA lights on my keys before and found them all to be too large. The eGear is much smaller and roughly as bright as something like a Fenix e01 (10 lumens). The biggest issue is the use of LR41 batteries. The little button cells are fine as long as they're working, but as you can see the ones in my light are just about done, barely lighting the Nichia emitter. I got about 2 years of use out of them, though, but they're way less convenient than other formats. These batteries are not always easy to find, and if you resort to buying them at a drug store, be prepared to spend what you did to buy the light in the first place. I still like the light, but now that the batteries are dead I'll likely replace it with something else.
I've also got a cheapo titanium carabiner on there, which is just for clipping to my belt loop or a bag strap, etc. It's not fit for load bearing so it's got limited uses, but it's still nice to have.
This is really the maximum of what I will permit myself to put on a keychain, and even this is pushing it. But I'm always looking for ways to reduce even more.
Got any ideas or setups that have worked for you? Let me know in the comments, I'd love to hear them.