MTG Salvation

Cycling lands (Barren Moor, Lonely Sandbar, Tranquil Thicket): Replace themselves early for a relatively low cost, and can get brought back from the yard later in the game. I opted for the Onslaught cycling lands rather than the original Urza's Saga cyclers or the Amonkhet deserts because these cycle for the cheapest, and I already run way too many lands that enter the battlefield tapped.

Blighted Cataract: I originally had Memorial to Genius here, but the fact that the Cataract enters the battlefield untapped and can be cracked immediately beats out the fact that it taps for colourless and costs one more to activate.

Drownyard Temple: Taps for colourless, but can be brought back easily, especially useful as pseudo-ramp in turns four to six.

Gargoyle Castle: This deck likes its games long and grindy. Generating a 3/4 flyer every turn fits that strategy.

Myriad Landscape: Recently became budget with its M25 printing. This will dig up two forests 95% of the time, but it's reusable and it thins your deck.

Elephant Grass: Propaganda isn't budget. This is. You can easily skip the cumulative upkeep, then recast it with Muldrotha out. If you need to cast another enchantment that time, the cumulative upkeep isn't so punitive that it can't be paid for two or three turns.

Hope of Ghirapur: Once it connects, it can easily lock that pesky spellslinger or superfriends player out of the game entirely. On top of that, you get some extra versatility because it can be cast off of Muldrotha as either an artifact or a creature.

Evolutionary Leap: Lets us find new creatures to (ab)use while recycling enter the battlefield triggers, at almost no cost.

Wood Sage: Occasionally I'll tap this naming something like Sakura-Tribe Elder, but mostly it's there to make sure I have a nice and juicy graveyard waiting when Muldrotha makes her dramatic entrance.

Sultai Ascendancy: It smooths out your draws, and under the right circumstances it can be a onesided Howling Mine.

Zur's Weirding: Hooo boy, when this card sticks around things get weird. Muldrotha lets you play around the drawback (other than when your instants and sorceries get dropped, but we have recursion for that), but it will make games very chaotic.

Nihil Spellbomb: Even if there's no relevant graveyards to eat, it's still drawing a card every turn for 1B.

Loaming Shaman: Can shuffle back stuff from your own graveyard if necessary, or an opponent's graveyard if you don't have access to the spellbomb.

Horizon Spellbomb: It's repeatable colour fixing with optional card draw stapled on.

Khalni Heart Expedition: Surprisingly repeatable, there's several cards in this deck that can drop multiple lands in a single turn.

Splendid Reclamation: When you get this in your opening hand you can easily create situations where you suddenly untap with eight to ten lands available when you're ready to cast Muldrotha for the first time, meaning you can start generating value the moment she comes down.

Epiphany at the Drownyard: A big Fact or Fiction-esque mana sink.

Vessel of Nascency: Repeatable self-mill.

Dreamborn Muse: In long, grindy games, this can help speed an opponent along to death by milling, and it fills your own graveyard. It's also a creature, meaning it can be quite hard to get rid of.

The Umbras (Crab Umbra, Spider Umbra): Cheap protection against the most common forms of removal. Can also easily be repeated.

Siren Stormtamer: Not only protects Muldrotha from targeted removal, but also your graveyard from getting exiled, as the majority of effects like Tormod's Crypt require the player to be targeted. Also, re: the deck's budget, this was at $0.96 in the first draft, but it more than doubled in price since then.

Jarad, Golgari Lich Lord: Comes down late in the game as a big body that can take large chunks out of your opponents' life totals.

Xathrid Demon: Risky, but it's a large evasive body that does the same as Jarad late game.

Torgaar, Famine Incarnate: Can repeatedly set you back to 20 life when you're being focused by the table, or it can put that pesky Oloro player into a manageable state.

Sinister Concoction: Repeatable removal for a negligible cost. Three quarters of the cost of activating it are things that Muldrotha actually wants to happen.

Nevinyrral's Disk: When paired with an umbra this can pretty much lock opponents out of developing any kind of serious board state.

Implement of Examination: A repeatable draw two for 3U.

Slithermuse: Can be Evoked from the graveyard for repeatable refills on your hand.

Tatyova, Benthic Druid: Generates great value off of cards like Splendid Reclamation and Myriad Landscape

Codex Shredder: It's mostly there to get back any instant or sorcery for [cost]6[/cost].

Den Protector: It's mostly there to get back any instant or sorcery for [cost]4G[/cost].

Jarad's Orders/Final Parting: Both do pretty much the same, essentially tutoring two cards. Final Parting costs one more, but lets you get any card.