CCP is launching the Dominion free expansion to EVE Online today. Dominion changes the way territory is conquered in the space MMO, increases players' control over space they own, makes changes to capital ships, upgrades planetary and starfield graphics and more.
You can read more about the features at the official Dominion site. Elsewhere on the site, you'll find exhaustive full patch notes (although at time of writing this part of the EVE website is down for maintenance).
The aim behind the changes to the system of territorial control - known as Sovereignty - is to make the struggle between player Alliances for sections of space more tactical and less "grindy", and to require active defence of captured space.
Hand-in-hand with the changes is a new system for upgrading the infrastructure in your sovereign space, with more options for changing resources and events, giving more reasons to actually occupy space you've won.
Capital ships have been rebalanced with new Doomsday Devices, tools for teaming up and running a fleet are improved, there's a new epic mission arc for the pirate faction, planets and starfields have been given a fresh lick of texture with some beautiful new procedurally-generated graphics, the in-game mail system is finally bearable, there's a new in-game browser based on Google's Chromium, an in-game jukebox with MP3 support and a ton more.
Incidentally, the significance of releasing Dominion today is that it's the day Iceland celebrates its own sovereignty. Happy birthday Iceland!