My favorite little powergaming rule is Core. I present to you the Shadowdancer's Hide in Plain Sight ability from the DMG:
SRD said:
Hide in Plain Sight (Su): A shadowdancer can use the Hide skill even while being observed. As long as she is within 10 feet of some sort of shadow, a shadowdancer can hide herself from view in the open without anything to actually hide behind. She cannot, however, hide in her own shadow.
Make a Wood Elf rogue/ranger/fighter/wild runner/scout/shadowdancer/dread commando with the power attack and spring attack chains and wield a glaive. You can Hide in Plain Sight (no cover needed, they can even be observing you), keep hiding on the way in of you spring attack, make a two-handed power attack with your glaive for full sneak and skirmish damage, maintain the Hide while attacking (at -20 penalty, very doable with max ranks Hide, elven cloak, and Wood Elf dex), and maintain the Hide while on the way out of your spring attack (at -5 penalty).
With your base speed of 50 (30, plus 10 for scout, plus 10 for wild runner--not counting Boots of Striding or spells) and your 10' reach, you'll wind up far away. You'll do massive damage. And your opponent will literally not know what hit him (you were hidden the entire time).
Only requirement is that the whole time you have to be within 10 feet of "some sort of shadow".
So, make sure there's a bright source of light (lantern, cloudless sun, sunrod tossed earlier, etc.) and that your incoming and outgoing path stay within 10' of your opponent, your mount, your allies, a tree, a cart, a building, a desk, whatever. All these things cast shadows. Heck, take a look around you right now (while at your computer) or walking outside--you'll see shadows all over the place.
Pretty neat. And very effective.
Best part is that it requires only four feats (Dodge, Mobility, Spring Attack, Power Attack), leaving plenty of room for flavor feats or augmentations (like the spring attack feats from PHB II). And it has tons of skills and fun/useful class abilities. I've played this character in Living Greyhawk from 1st level to 12th and had a great time at every level.
-z
* Wild Runner is from Races of the Wild, Scout and Dread Commando are from Complete Adventurer. If those are unavailable, take more Rogue levels or other class that gives more Sneak Attack damage.