Labyrinth Lord is a brilliant retro clone that takes you back to the old days of D&D (something I would know nothing about, I'm the Wizards generation). If you play LL then Matthew over at The Dwarf and the Basilisk has a little gift for you in the form of pre-rolled NPCs that can be used as henchmen or party member replacements.
I love the back to basics feel of Labyrinth Lord, something that's refreshing when I've been playing 3e for so long. Not that 3e is bad or anything, it's incredibly enjoyable, but it's a complex system with so many supplements to choose from my head was spinning when I went into a games store. LL is the gritty dungeon crawl of yesteryear.
Retro clones are becoming ever popular on kitchen tables and one that I really like is Mazes and Minotaurs, which is completely free and well worth a try. It's D&D meets Jason and the Argonauts, which can only be all kinds of cool. I'll probably talk about it more in a future post.
But if it's retro you're looking for then T&T wins hands down because it has never stopped being retro, it's stuck to pretty much the same formula since its conception. Sure there have been changes in level advancements and classes etc but at heart it's still the same great game it always was, and it's cheap too! For something now in its 7th edition I'd say that's an incredible feat.