Sliver Legion

All Sliver creatures get +1/+1 for each other Sliver on the battlefield.

Plains

Sliver Overlord

{3}: Search your library for a Sliver card, reveal that card, put it into your hand, then shuffle.
{3}: Gain control of target Sliver. (This effect lasts indefinitely.)

Swamp

Island

Groundshaker Sliver

Sliver creatures you control have trample. (A creature with trample can deal excess combat damage to the player or planeswalker it's attacking.)

Forest

Mountain

Sliver Hivelord

Sliver creatures you control have indestructible. (Damage and effects that say "destroy" don't destroy them.)

Galerider Sliver

Sliver creatures you control have flying.

Sliver Queen

{2}: Create a 1/1 colorless Sliver creature token.