I was big into retrocomputing from about '96 through '04. Lately, I haven't spent much time dabbling in this area.
What I'm still involved with:
Atari 8-bit. I'm down to one computer (plus spares), which is an 800XL that I hacked in the Super Video 2.1 upgrade, plus a firmware kit that enables me to switch between four different OSes. I use this, primarily, as a game machine. I also have some older game consoles (Atari 2600, Atari Jaguar, Nuon), which are all wired up to the same home theater via svideo.
Now that I think about it, I also have an Atari XEGS, which is technically an 8-bit computer, but it never gets used. In the past, I had had a few other 8-bits as well, like a 1200XL I had since 5th grade. I eventually got rid of them to reduce the clutter, as well as about 200 cartridges and lots of peripherals and controllers.
Tandy TRS-80 Model 100. If you don't agree that this is the coolest computer ever made, then you need to be re-educated. I'm on the Club 100 mailing list and a group of us (primarily under the guidance of one self-motivated person) make hardware upgrades for this puppy, to give it greater memory and storage capacity. I still have two of these, one of them stock (of which I'm the original owner, from when I got it as a birthday gift when I was sixteen) and one of them wired up with the memory/flash upgrade. I can't say I use this computer much, but I'll never get rid of it. It's just that cool.
What I used to be involved with:
Old SGIs. This is fresh from my post on the SGI keyboard thread under "keyboard audio clips" in a deja vu kind of way. In the world of retrocomputing, SGIs were my strongest passion. I was especially interested in the Indigo, and went through variants and hardware of all stripes. I had one machine in particular that I invested all of my time with, and tricked it out to the hilt. It was such a hot-rod, with so many rare components and original packaging, that when I sold it on eBay in 2005 (in an era when stock Indigos went for appx $50), It sold for $880.
I also had an R5k Indy, an R4600SC Challenge S, deskside Power Series 4D/310 VGX, a deskside Challenge L, and through dumb luck, I picked up four deskside Onyx RE2s from Boeing Surplus (sans hard disks, which were destroyed, to comply with confidentiality requirements) and fixed them up with new hard disks and fresh Irix installations, then sold them via eBay. This was simultaneously euphorically fun, and crushingly frustrating. (Wow, three adverbs in one sentence!) E.g., shipping a 250-pound computer isn't as easy as it sounds.
Old Suns. I particularly liked the old lunch boxes, in particular the Sparc IPX. I had one of these as a project machine for about two years, and even brought it to work to use as a development box. I also had an ancient 4/260 deskside server that I had hoped to do something interesting with (and a Sparcbook 2, now that I think about it), but never did. I just gave it away when I got tired of it taking up garage space.
NeXT and MicroVAX. I only dabbled in these areas, getting stuff up and working in a gee-whiz capacity, and then sitting there wondering why I had wanted to get one of these working in the first place.
Old Macs. This probably doesn't count as retrocomputing, but I tricked out a project SE/30. It had a full 32MB of RAM, a 4GB hard disk, and under the hood were a Sonnet 50MHz 030 upgrade and that cool graphics card that provides greyscale graphics to the internal display. Plus it quad-booted into four different OSes: System 6.08, System 7.1, a hacked version of Mac OS 8.1, and NetBSD.
Routinely, for years, until approximately 2001, I used a Mac IIci that was upgraded with an 040 expansion board and nubus graphics card, when it was finally replaced with a blue-and-white Powerbook G3. (At the end, I was frustrated with how unstable it was.)
Likewise, a Powerbook 100 was my primary laptop for six years (from '98 to '04), only to be replaced when I found something worthy to succeed it (a 12" Powerbook G4, which I still use today). I loved that thing. It and I traveled to six continents. Like the SE/30, it was also multi-boot, selectable between System 7.1 and System 6.08.