I have several systems in semi-regular use, both for fun and profit.
-IBM XT: Archiving and writing floppy images for my other vintage systems. Usually I'll end up browsing the forums with my shell account, and transferring files with FTP. There's a Panasonic dot-matrix printer hooked up to it, which I use to print text files and receipts when I don't want to waste my inkjet ink. It also sees use as a terminal for my Altair 8800, and for serial file transfer with ADT.
-Homebuilt 486: My main retro gaming rig and a tweener system for 3.5" and 5.25" FDDs. It runs a network share and a web server too.
-Toshiba T3100e and T3200SX: Programming in QBASIC, using as a terminal, and burning EPROMs.
I often get boxes of ISA cards from the recycler, so I use these systems to test them before listing them on ebay.
I've also bought an InterMedia system recently, which consists of X2 ISA cards and Windows 9x software to work with most floppy and tape formats. I'll soon be building another 486-PIII based system to work with this, and have it hooked up to 3", 3.5", 5.25", and 8" drives simultaneously, along with an assortment of SCSI tape drives. Maybe I'll get a data conversion contract and break even? Fingers crossed
Great answers guys! Thanks! Sidebar: when it comes to rarer systems, do you advocate using them lots (assuming you have a reason to), or minimal use to preserve? I often wonder how future vintage collectors will cope with 50, 100 year old machines with few or no spare parts available.
I think it depends more on the rarity of the compents than the entire system, for example, I wouldn't want to leave CRTs, disk motors, and machines with propietary ICs running too often (Amigas spring to mind), while on the other end, I wouldn't be too worried about running an Altair 8800 24/7, if you were to swap out the PSU for a more modern switch-mode supply. There's enough spares out there that it wouldn't suprise me if natural degradation would break all the remaining NOS ICs before they ran out.
Apart from that, I'd pretty much do what Paul suggests.