Here's what I've used before and what works and doesn't:
WD-40 - gets contacts clean and then coats them so they don't get dirty again as fast..doesn't take much either As far as I know..no side effects yet *crosses fingers* I like using it cause it makes everything look nicer and play easier IMO.
Goo Gone - Not only orangy fresh, but it also takes off all kinds of gummy sticker glue and it doesn't ruin glossy stickers (although cheap stickers stain with it). Good all around cart cleaner too. Even takes off SOME permanent marker
Goof Off - This shit is dangerous! If you use it..beware..it eats right thru plastic in seconds...liquifies it! One drop spread rapidly will melt the top layer of plastic right off of a cart, easily removing permanent marker, but if you're not careful, it can chemically stain or burn the cart, and/or ruin it. Not to mention the ink coated plastic tends to spread the ink around a bit so you're sometimes left with a darker splotch of plastic that needs heavy Goo Gone scrubbing. Use at your own risk! When I use it, I drop a drop or two on a cloth and then run it on the item...never use it directly on the game.
Perma-Spin - http://www.exlumen.com/ Awesome awesome scratch buffing liquid! Polishes CDs WITHOUT scratching them and often removes (ok it fills them invisibly!) scratches making scuffed games look better. Not sure how it works on PSX games with the black discs, but regular CDs like Sega CDs it works like a charm Great stuff!
Alcohol - Well...duh. Rubbing alcohol is the old stnadby for cleaning contacts, stickers etc. You all should know how it works. Great for simple jobs, but not very effective for lots of sticker glue or getting out pen or anything like that.
I personally like to do an initial cart cleaning with rubbing alcohol, then clean the contacts again with WD-40. That way all the dirt is gone and then the contacts are protected from more dirt for a while. For stubborn stickers, pen, or glue I use Goo Gone (it also polishes carts and stickers!), and for insanely nasty stuff I hesitantly use Goof Off (but not often!). Perma Spin is great for nasty discs, but if your discs only have one or two minor scratches you're better off not using it. Anyone else have recommendations for game cleaning solutions?