Sets released: Morningtide, Shadowmoor, Eventide, Shards of Alara

Major stuff:

   - Shards of Alara made a move to a smaller set size and introduced
     mythic rare cards, approximately twice as rare as rare cards.
   - The concept of linked abilities was widened and got its own section
     in the rulebook, marking the end of shenanigans with cards like
     Callous Oppressor.

Minor stuff:

   - The abilities prowl, reinforce and kinship appeared in Morningtide;
     conspire, wither and persist in Shadowmoor and retrace in Eventide.
     Shards of Alara brought exalted, unearth and devour.

   - Shadowmoor introduced the untap symbol {Q}.

   - Shadowmoor introduced costs that are paid with a number of
     mana of any color or just one mana of a particular color, e.g.
     {2/B} can be paid as {2} or {B}.

   - Shadowmoor gave the game another multicolored artifact, the Reaper
     King.

   - Shards of Alara continued the colored artifact theme in earnest.