Urborg, Tomb of Yawgmoth

Each land is a Swamp in addition to its other land types.

Filth

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)
As long as Filth is in your graveyard and you control a Swamp, creatures you control have swampwalk.

Blistergrub

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)
When Blistergrub dies, each opponent loses 2 life.

Zodiac Monkey

Forestwalk (This creature can't be blocked as long as defending player controls a Forest.)

Zodiac Rabbit

Jarad, Golgari Lich Lord

Jarad, Golgari Lich Lord gets +1/+1 for each creature card in your graveyard.
{1}{B}{G}, Sacrifice another creature: Each opponent loses life equal to the sacrificed creature's power.
Sacrifice a Swamp and a Forest: Return Jarad from your graveyard to your hand.

Lynx

Eladamri, Lord of Leaves

Other Elf creatures have forestwalk. (They can't be blocked as long as defending player controls a Forest.)
Other Elves have shroud. (They can't be the targets of spells or abilities.)

Stalker Hag

Swampwalk, forestwalk (This creature can't be blocked as long as defending player controls a Swamp or a Forest.)

Cabal Coffers

{2}, {T}: Add {B} for each Swamp you control.

Life and Limb

All Forests and all Saprolings are 1/1 green Saproling creatures and Forest lands in addition to their other types. (They're affected by summoning sickness.)

Raiding Nightstalker

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)

Evil Presence

Corpse Harvester

{1}{B}, {T}, Sacrifice a creature: Search your library for a Zombie card and a Swamp card, reveal them, put them into your hand, then shuffle.

Plague Beetle

Bog Raiders

Quag Vampires

Multikicker {1}{B} (You may pay an additional {1}{B} any number of times as you cast this spell.)
Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)
Quag Vampires enters the battlefield with a +1/+1 counter on it for each time it was kicked.

Marsh Boa

Anaconda

Bog Tatters

Blanket of Night

Jedit Ojanen of Efrava

Forestwalk (This creature can't be blocked as long as defending player controls a Forest.)
Whenever Jedit Ojanen of Efrava attacks or blocks, create a 2/2 green Cat Warrior creature token with forestwalk.

Warthog

Crypt Ghast

Extort (Whenever you cast a spell, you may pay {W/B}. If you do, each opponent loses 1 life and you gain that much life.)
Whenever you tap a Swamp for mana, add an additional {B}.

Jukai Messenger

Tourach's Chant

At the beginning of your upkeep, sacrifice Tourach's Chant unless you pay {B}.
Whenever a player puts a Forest onto the battlefield, Tourach's Chant deals 3 damage to that player unless they put a -1/-1 counter on a creature they control.

Wormwood Treefolk

{G}{G}: Wormwood Treefolk gains forestwalk until end of turn and deals 2 damage to you. (It can't be blocked as long as defending player controls a Forest.)
{B}{B}: Wormwood Treefolk gains swampwalk until end of turn and deals 2 damage to you. (It can't be blocked as long as defending player controls a Swamp.)

Leaf Dancer

Nirkana Revenant

Whenever you tap a Swamp for mana, add an additional {B}.
{B}: Nirkana Revenant gets +1/+1 until end of turn.

Zodiac Rat

Roots of Life

As Roots of Life enters the battlefield, choose Island or Swamp.
Whenever a land of the chosen type an opponent controls becomes tapped, you gain 1 life.

Bog Smugglers

Chatterfang, Squirrel General

Forestwalk (This creature can't be blocked as long as defending player controls a Forest.)
If one or more tokens would be created under your control, those tokens plus that many 1/1 green Squirrel creature tokens are created instead.
{B}, Sacrifice X Squirrels: Target creature gets +X/-X until end of turn.

Shanodin Dryads

Somberwald Dryad

Thelon's Chant

At the beginning of your upkeep, sacrifice Thelon's Chant unless you pay {G}.
Whenever a player puts a Swamp onto the battlefield, Thelon's Chant deals 3 damage to that player unless the player puts a -1/-1 counter on a creature they control.

Cat Warriors

Rushwood Dryad

Mire Boa

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)
{G}: Regenerate Mire Boa. (The next time this creature would be destroyed, instead tap it, remove it from combat, and heal all damage on it.)

Bog Wraith

Scarwood Bandits

Forestwalk (This creature can't be blocked as long as defending player controls a Forest.)
{2}{G}, {T}: Unless an opponent pays {2}, gain control of target artifact for as long as Scarwood Bandits remains on the battlefield.

Mirri, Cat Warrior

First strike, forestwalk, vigilance (This creature deals combat damage before creatures without first strike, it can't be blocked as long as defending player controls a Forest, and attacking doesn't cause this creature to tap.)

Yavimaya, Cradle of Growth

Each land is a Forest in addition to its other land types.

Magus of the Coffers

Willow Dryad

Cabal Stronghold

{T}: Add {C}.
{3}, {T}: Add {B} for each basic Swamp you control.

Pygmy Allosaurus

Ashaya, Soul of the Wild

Ashaya, Soul of the Wild's power and toughness are each equal to the number of lands you control.
Nontoken creatures you control are Forest lands in addition to their other types. (They're still affected by summoning sickness.)

Hag Hedge-Mage

When Hag Hedge-Mage enters the battlefield, if you control two or more Swamps, you may have target player discard a card.
When Hag Hedge-Mage enters the battlefield, if you control two or more Forests, you may put target card from your graveyard on top of your library.

Bayou Dragonfly

Flying; swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)

Living Lands

All Forests are 1/1 creatures that are still lands.

Sheoldred, Whispering One

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)
At the beginning of your upkeep, return target creature card from your graveyard to the battlefield.
At the beginning of each opponent's upkeep, that player sacrifices a creature.