Every system I have that is
capable of booting into OS 9 has it installed, although some have been upgraded to the point where OS 9 doesn't run well. As a prime example, I put a hacked Radeon 9600XT video card in my main Quicksilver(2002, dual 1ghz). The 9600 supports core image, which make Tiger run great and Leopard really rock, but at the cost of any video acceleration in OS 9. I'm also running a PCI FX5200 in my G4-upgrade B&W, which locks it out of OS 9(I'm going to get Leopard installed on it one of these days)
Almost all of my G3 systems run OS 9 primarily, although most of them have Tiger installed. I say "almost all" because I have a B&W that runs a couple of versions of Rhapsody and all the OS X DPs.
My lower spec G4s also run OS 9 primarily, although I tend to also install Tiger and/or Leopard on them. I have three Cubes-a factory 450mhz, a factory 500mhz, and one with a Sonnet 800mhz upgrade. The 450 and 500 are primarily OS 9 machines, while the 800 is a Leopard machine.
I also have a G4 Gigabit Ethernet(dual 500) running OS 9 as well as every compatible retail version of OS X from Public Beta to 10.5.8.
I tend to put at least decent OS 9 compatible video cards in AGP G4s that will run OS 9. Two of my Cubes have GEForce 2MXs in them, while the third has a Radeon 7500. Once I get around to flashing it, I have a GEForce 6200(core image support) that's going in the 800mhz. I'd like to get a GEForce 3 for one of the others, as the 2MX runs uncomfortably hot in the Cube for me(even with a base fan).
I also have a blast running it on TiBooks. My 400mhz only has OS 9, while my higher spec ones(867mhz and 2x 1ghz) dual boot Leopard and OS 9. These computers are actually surprisingly fast with Leopard, and are rockets with OS 9.
I'm running GEForce 4MXs in a couple of computers, and Radeon 9000s in a few others. The 9000 is supposedly better performing, but you have to remember to install the drivers
before installing the card or OS 9 will lock up at the desktop(the special MDD OS 9.2.2 build doesn't have this problem as this was a stock card). My Sawtooth has one of the super-rare Mac Edition Radeon 8500s. My last GEForce 4Ti died, but I have another on the way. I'm not sure where it's going to end up-possibly in the GigE.
I like OS 9 on higher spec OWR systems(604Es and G3s), so am currently running it pretty much exclusively on my Beige G3s, 8600, and 9600. I also have a 7350 WGS that I haven't even booted up yet

, and don't know what it has installed. My 8500 is running OS 8, but I probably should upgrade to 8.5. I have Rage 128s in all of my PCI OWR Macs.
My 8600 is undergoing a bit of a project at the moment. I recently installed a Sonnet G4 card at 700mhz with 1mb of L3 Cache. This one has a Radeon 9200 and an IDE Card. I'm planning on installing Leopard on it when I have the time(I need to get more RAM and a USB/Firewire card).
I have a "hotrod" G3 desktop in which I installed the 400mhz processor that I removed from my G4 upgraded B&W. Thanks to 66mhz system bus on the beige boxes, I actually ended up overclocking it slightly to 433mhz. It has a Rage 128 along with a USB and Firewire card. With 768mb of RAM(the max) it's very peppy. When I get into full-time teaching this summer, I'm probably going to use it as an exam writing computer.
I briefly upgraded my Quadra 700 to OS 8, but ended up reverting back to 7.5.5. My SE is currently running System 6(what's installed on the HD20 that I currently boot it off of). Once I get around to replacing the internal drive, I'm probably going to install System 7.