With the resurgence of D&D and the popularity of the B/X edition set of simpler rules, a number of old school players have created retro-clones to meet the new demand for a cheap set of printed rules that can be referenced in new adventures.
Their retro-clones remix, reformat and put a new spin on the B/X edition D&D material.
One such retro-clone is the Basic Fantasy Role-Playing Game or BFRPG.
The Basic Fantasy RPG manual created by Chris Gonnerman is offered as a free download, or as print on demand on Lulu.com or for purchase on Amazon. Chris has reformatted the B/X rules into an easier to use format and offers an affordable alternative for those who want a printed hard copy for every player.
This one manual covers characters from level 1 to 20, in dungeon and wilderness adventures.
...but that is only the beginning.
A lot of add-ons for free
The BFRPG web site houses every permutation of optional rules (add-ons) you can imagine and the forum ensures that each optional rule set undergoes some level of rigor to keep them on par with the same level of quality/format as the BFRPG.
And the best part, it is all free of charge.
In terms of staying true to the original B/X edition, BFRPG presents many modifications such as separating Race from Class (good), and using an Ascending Armor Class instead of Descending (who cares), and offers quite a bit of 'homebrew' or 'house rules' as optional content in addition to stating the original (it is optional).
Players that want use more 'advanced' classes will find optional add-ons to suite their need without unbalancing the game.
Players that feel they need only the old school classes can ignore the add-ons altogether.