I have this folder in my Dropbox account: RPG/Tablet. It contains copies of all the PDFs that I keep synced to my tablet, using the GoodReader app (which is mandatory for reading PDFs if you have an ipad). Here are the ones that can be downloaded free of charge. (Note that “free of charge” does not imply unencumbered by copyright.)

• AD&D Dungeon Masters Guide Appendix A Random Dungeon Generation. Gygax’s original random spur for generating dungeons. This free sample was made available as a promotion for the recent AD&D premium reprint.
• Akratic Wizardry. Swords & Sorcery house rules, including damage by class, wisdom-based sanity, and a colors of magic system that collapses the magic-user and cleric into the magician class. HTML index of rules here.
• Better Than Any Man. An extensive historically based early modern sandbox setting with time based events, numerous lairs, and lots of new content (spells, monsters, etc). This was the LotFP offering for Free RPG Day 2013, and is now available pay what you want.
• Campbell – DM1 Treasure. Help generating memorable treasure. Leave Courtney a word of thanks for this great resource here.
• Campbell – DM2 Tricks, Empty Rooms, & Basic Trap Design. Some tables to help you populate rooms without monsters. Leave Courtney a word of thanks for this great resource here.
• Castles & Crusades Quick Start. A low-level intro to Castles & Crusades, one of the first OGL “clone” games, incorporating many Third Edition innovations (such as ascending AC) into a tight game somewhat reminiscent of AD&D. C&C was integral to the formation of the OSR, and the C&C PHB may be the best selling OSR product so far.
• Challenge of the Frog Idol. This module is a swamp-crawl by Dyson Logos and features a raft made of zombies. What else do you need to know? Overview page.
• Chandler – EGG Teratic Tome. Even though I’ve included this in the free resources list, technically it is pay what you want, and as one of the best RPG bestiaries released so far in any form, it’s probably worth throwing some cash in Rafael’s direction.
• Conley – Blackmarsh. A free setting with keyed hex map and starting town. Also includes some custom rules connected to the setting, such as “viz” (solidified magic). Release post here.
• Crawford – Stars Without Number. An sci-fi game with extensive support for setting generation and a rules-lite approach to character classes.
• DCC RPG Quick Start Guide. Includes random birth augur, equipment, and occupation tables.
• Delta House Rules. An elegant core mechanic approach to classic D&D. Includes a simple fighter feat system for added customization.
• Dyson’s Delve Deluxe. A vertical mega-dungeon consisting of individually small levels, fully keyed and quite usable. Summary page here.
• Labyrinth Lord. OGL clone of Basic/Expert D&D (incorporates several d20 SRD-isms). Has become the lingua franca of flailsnails. The pay version has an extremely tight aesthetic due to the art of Steve Zeiser. Overview page here.
• Labyrinth Lord Advanced Edition Companion. An AD&D approach to Labyrinth Lord, with added classes, races, spells, and so forth.
• LotFP – Rules and Magic 2013. Highlights include the tweaked magic (see the summon spell) and a recently added appendix with firearms rules.
• Metamorphica. An entire book dedicated to tables for mutations. The softcover version available from Lulu is also quite attractive. A necessity for any devotee of chaos magic.
• Monsters of Myth. A free, illustrated bestiary for OSRIC.
• Mutant Future. Inspired by the original Gamma World and mostly compatible with Labyrinth Lord (and thus many other OSR clones). Wacky HP rules, but useful for mutation tables and apocalyptica. Summary page.
• Philotomy’s Musings on OD&D. The best introduction to and discussion of the original Dungeons & Dragons game. Some excellent house rules suggestions as well. Conveniently transcribed into HTML by Ramanan S.
• Quick Primer for Old School Gaming. Rulings, not rules. Player skill, not character abilities. Heroic, not superheroic. Forget game balance. Read this to get a sense what many people are talking about when they mention “old school” D&D.
• Rients – Miscellaneum of Cinder. Quoting from the author’s description: “an ill-conceived collection of random dice charts vaguely compatible with games involving dungeons, labyrinths, swords, spellcraft, wizardry, lords, dragons and etcetera.”
• Rients – Under Xylarthen’s Tower. A wonderful little archetypal intro dungeon of four levels.
• Rients – What Went Wrong. Best random character creator ever? Very likely. Yes, the link is to a Google Doc. But! File -> Download as -> PDF.
• Shear – TOTGAD. The gothic career system is the highlight of this collection for me, though there are many other useful tools, a gothic “Appendix N” (Jack is a professor of Gothic literature), and 13 variations on gothic settings. Also requires you to do the download as PDF thing. Release page here.
• Swords & Wizardry WhiteBox third printing. Full LBB-inspired rules chassis. Particularly well suited for use as the basis for custom systems incorporating house rules. Free version includes all the art, and the cover of the print version by Mullen is one of my favorite pictures to come out of the OSR. Also a good intro to the “imagine the hell out of it” ethos. Overview page.
• Telecanter – Magic Item Spur. Inspiration for creating unique magic items. This is one of my most used random tables. Summary post.
• Theorems and Thaumaturgy. A Labyrinth Lord OSR equivalent to the Tome of Magic. Contains new classes (elementalist, necromancer, vivimancer, fey elf, expanded illusionist) with full custom spell lists and numerous custom tomes (such as the The Chronomancer’s Workbook). All text is OGL and thus reusable in your own OGL works.
• Traveller Starter Edition Core Rules. Everything you need to play Traveller, one of the original 2d6 systems. Features a capable life path system as well.
• Varlets and Vermin. A collection of low level monsters to add variety to a campaign by Roger from the Roles, Rules and Rolls blog. Google Docs link, so do the download as PDF thing. Release post.
• World of Dungeons 1979. The fake progenitor of Dungeon World. Very short (only several pages) simplified version of DW. Check out the excellent magic system. Also features a page of S&S themed names.

That’s a lot of pretty amazing free resources. You could play for the rest of your life, probably, with nothing else.

I’m sure there are many other free resources, too! This list is not intended to be comprehensive. It is what I actually keep synced to my tablet all the time.