This is a list of combos and synergies I've found that have unique and interesting effects. If one of them inspires you to brew something up, please drop a link in the comments--I want to see what you've done. Also, please comment if I got something wrong. I don't want to be spreading bad combos.
Grave Danger
This is an expansion of Haakon, Stromgald Scourge + Rooftop Storm, which allows you to cast Zombie Knights from the graveyard for free. Tresserhorn Skyknight enters for free and gets Dragon Breath. Changeling Berserker would also work, but you need another creature to Champion. Nameless Inversion clears out blockers, and Blades of Velis Vel makes your creatures as hard-hitting as they need to be.
Cat Recruitment
If your opponents have creatures, this gets ridiculous fast. When Nacatl War-Pride attacks with Blade of Selves, it copies itself once for each creature the defender has and once for each other opponent. Those tokens normally go away at the end of your combat step, but Sundial of the Infinite keeps them around to copy themselves more next turn.
Sphinx Jump
Enchant Sphinx Summoner with Infinite Reflection. Play a 0-cost creature, which enters as Sphinx Summoner. Search up a 0-cost artifact creature, and repeat. This also works with Rune-Scarred Demon.
I Keep It in the Basement
Graveyard? Battlefield? What's the difference? Oh, check out my sweet zombie collection, too! It doesn't matter what the mana cost of your creatures is when you keep bringing them back for , payable in installments.
Broken Mirror
Mirrors are scary things. They say your soul can get trapped in a mirror. If you cast Fractured Identity on Lich's Mirror, each opponent creates their own Mirror. If you take them all with Blatant Thievery, because a token's owner is the one who created it, you won't lose until all the Mirrors, or all your opponents, are gone. When you lose and reshuffle, because you control the Mirrors without owning them, they stay in play.
Wind
: Each other player returns a permanent they control to their hand. The Darksteel Relic can be switched with any 0-cost artifact. Pay for Words of Wind's ability, then drop your artifact. This triggers Riddlesmith (or Vedalken Archmage)'s draw ability, which triggers Words of Wind. Return your artifact and repeat as desired.
Dances with Bears
: Create a 3/3 green Bear token and scry 1. Similar to Wind, this relies on triggers triggering triggers. Activate Words of Wilding to replace your draw with a bear. Sylvan Anthem allows a scry and pumps the bear enough to trigger Elemental Bond, which allows you to draw, which can be replaced by a bear. Ayula, Queen Among Bears likes this.
Legends on Demand
, Sacrifice three creatures: Search your library for a legendary creature card, put it onto the battlefield, and shuffle your library. Aww yeah. You can spray Spiders everywhere with Ishkanah, Grafwidow. You can pull out Celestial Kirin and start blowing stuff up. Or, you can turn Raise the Alarm into the ultra-deathinator of your choice at instant speed. Go wild.
Island Vacation
, Discard a card, sacrifice a land: Search your library for four basic lands and put them onto the battlefield, then shuffle your library. Surround yourself with Island luxury. The efficiency of this combo can be hugely increased with cards like High Tide, Training Grounds, or Extraplanar Lens, and one of these plus Seer's Sundial means you're not limited by discard.
Hammer Ward
Give your opponent piles of artifact enchantment creature tokens while sacrificing your lands to restrict the number of permanents they can play. Of course, those tokens can still attack, be sacrificed, and otherwise be cleaned up, but that's what the rest of your deck is for.
Unkillable Death
"When this creature would die, return it to your hand, then put it onto the battlefield." Stack the triggers so Mortus Strider resolves before Deathrender, and it's pretty much got indestructible. It also works with Endless Cockroaches, which are widely revered and worshipped for their ability to not die, but are more worrisome when they start carrying swords.
Creative Thinking
At the beginning of your upkeep, surveil 1, then draw three cards. You may play two additional lands each turn. Whenever you cast a spell, draw two cards. You may exile cards from your graveyard to pay the colorless cost of spells. At the end of your turn, discard your hand. Sound like fun?
From Beyond the Sky
Big spells! Chaos! Dinosaurs and Phyrexians! Whenever Pyrohemia deals damage to Ripjaw Raptor and Rona, Tolarian Obliterator
, you get to draw a card, then play a random card from your hand for free. Moonmist can set the scene, and Pestilence can fill in for Pyrohemia if you're not worried about Modern legality.
Dark Energy, Dark Matter
Infinite as Priest of Gix and Chthonian Nightmare create just enough resources to support each other leaving and re-entering play. Start with the Myr in your graveyard and Nightmare in play with all three energy available, then cast the Priest and sac it to ring back the Myr. Recast Nightmare, sac Myr for Priest, and repeat.
