Out of the Tombs Out of the Tombs earns a spot on this list as my slight favorite from a group of very similar cards (something which will also be true of several other entries)

These are some of the best ones I could find: Sin Prodder, Twilight Prophet, Combustible Gearhulk, Sorin, Grim Nemesis, Reviving Vapors, Rush of Knowledge, Ugin's Insight, Interpret the Signs, Nissa's Revelation, Greater Good, Demonlord Belzenlok, Emrakul's Influence Card Draw: Yidris, Maelstrom Wilder, Maelstrom Nexus, Sunbird's Invocation Cascades: Scattering Stroke, Psychic Battle Denial: Timesifter Extra Turns: Favor of the Mighty Protection: Recross the Paths, Selvala, Heart of the Wilds Ramp: Mercurial Chemister, Wolf of Devil's Breach, Riddle of Lightning, Blast of Genius, Disaster Radius, Heretic's Punishment, Kozilek's Return, Nahiri's Wrath, Displacement Wave, Gaze of Granite, Pernicious Deed Removal: Vengeful Rebirth Recursion: Fierce Empath, Treasure Mage, Conduit of Ruin, Grozoth, Sanctum of Ugin Tutors: Spellbound Dragon, Metallurgic Summonings, Kalemne, Disciple of Iroas, Vial Smasher the Fierce, Baneful Omen, Infernal Genesis Beatdown: I'm happy to see that Red is bringing the most value in terms of high cmc support cards

Generating one or more can be very useful if your deck includes cards that care about the number of artifacts you have
Do you need help with card draw for your Commander deck, and want to try out some colorless cards
No one really knows how much it will be worth, or even if it will ever be found
What Is Offspring