Sled drags provide a crazy quad pump and can do wonders for conditioning and leg growth. T Nation coach Christian Thibaudeau has even had competitive bodybuilders use them for leg size.
Bonus: They're also one of the most knee-friendly exercises and they provide a quad-building alternative to those with cranky backs.
There are more cheap sleds on the market than ever, so it's hardly an excuse any more if your gym doesn't provide one. There are nylon sleds safe for gym floors and durable metal sleds costing $100 that you can drag up and down the street. A sled and some Craigslist weight plates are a worthwhile investment.