I've found that borax (dissolved in warm water, or used with a wet sponge as a sort of polishing-compound) is excellent for cleaning up yellowed/scuffed plastic casings. Probably wouldn't be so hot on anything darker than C64-brown.
Lubit-8 is awesome for most moving parts, available at Advance Auto stores in the US. Don't try using it on the actual surfaces of floppy read-heads, though! (I did soak a bead of it into an aging foam seal on a Quantum HD, and that hasn't failed yet.)
WD-40 is good stuff, too- if you know what it was invented for. It's *not* a lubricant, it's a water-displacer. After you've washed your mainboard (or put your Saturn V through a rainstorm), do a decent job of shaking it dry, and empty a spray-can over it. The WD-40 will 'float' the water above itself, where it can evaporate without corroding the metal. (That's why it smells so strongly- it's volatile, unlike a proper oil like CRC or the Tufoil products. If you use it as an oil on a dying CPU fan or a squeaky hinge, it'll all be gone in a week, and your problem will be back.)
I'd be careful using H2O on anything with an integrated NiCd, like an A2000 board- those batteries have vents hidden somewhere on/in them, and liquid intrusion probably isn't a Good Thing as far as leakage is concerned. Otherwise, water immersion isn't too damaging if there's no current running- I don't know about "new motherboard smell," but I was under the impression that most manufacturers had transitioned to water-based flux by now, meaning your new board *may* have had a rinsing dip (in purified/deionized water) already!
