This is not really a dig at Retro-Clones, I actually enjoy Labyrinth Lord and Swords & Wizardry. But one game I keep going back to is Tunnels & Trolls! The nice thing about T&T is that it is still owned by the original author, the Ken St. Andre (a.k.a. The Trollgod) and it's still published by the original publisher Flying Buffalo owned by Rick Loomis.
T&T is currently in it's 7.5 version of the game, but was completely written by Ken St. Andre. The 7.5 version is backwards compatible with older versions of the game, but the current version just has a few more additions that in my opinion really add a lot to the game (spite damage and skills). T&T remains the old reliable rpg, in the sense that you don't have many versions that have been published by different authors with different interpretations. Everything has remained the way the author intended it to be! Plus it's one of the few RPGs that has solitaire adventures!
As far as online support, T&T has some great resources for Q&A and ideas for gaming. You get to interact with the Trollgod himself over at Trollhalla (I admit I haven't been as active as I should over there), explore the depths of Gristlegrim's Floating Fortress, or you can browse through some great game ideas over at Vin's Trollbridge, not to mention Troll bridge's free Trollszine and you can download free adventures over at Free Dungeons. Plus some great blogs like Troll Hammer's creature features, Alderwood Cemetary, Take That You Fiend, Trrrommms Weebly, Lone Delver, H'rrrothgarrr's Hovel and The Omnipotent Eye.
For other publishers besides the wonderful products over at Flying Buffalo, check out the T&T Lulu Page, Perryton Publishing, Jeff Freels blog, Jeff Werx is artist and creator of Bean! and Lone Delver Games.
But most of all, I love the humor and lightheartedness of T&T. I myself am basing my gaming world (actually city) Imajapoor, in Trollworld. If you're a fan of Discworld, Xanth, Wuntvor, and Robert Aspirin's "Myth" series you'll love gaming in Trollworld! So what are you waiting for!