Answer
Codex: Space Marines was released on the 4th October 2008
New special rules and different units
There are rumors of the new Codex:Chaos Legions coming out shortly after the 6th edition rules are released.
About a year ago. Space-wolves about 6 months ago. Blood Angles about 3 months ago. But if Games Work Shop starts to see sales drop, they will release a new space marine codex. Obis
AnswerThere are more guys and the armor, attack and defense of some of the units improves.
Space Marines are the most popular faction, within the Space Marine faction the Ultramarine chapter are the most used as they are Games Workshops poster boys. After the Space Marines it would be Chaos Space Marines and Orcs then Imperial Guard especially after the release of their new codex. Tyranids will have a new codex in January so their popularity may rise and by far the least played faction would be the Dark Eldar, this is because they haven't had a new codex or and significant update since about 1998
The latest Imperial Guard codex was released on May 2nd 2009
October 2009
see now me and my friends are in a debate about this about this because i have a dark angels army and he thinks i cant use the cannon and new assault squad and such but the way i see it is that the latest space marine codex refering to ALL space marines should be the one consider unless your own men have individual abilities (input from its the fuzz below) Frankly i have the new codex, the BA codex, BT codex and access to the SW and DA codexs. They say the blood angles codex is 5th edition and up to date. Some things need to be sorted but heres a few points, Scouts cost more can only come in 5 or 10 man squads have to pay for sniper rifles but have a higher BS, Assualt marines have options to better weapons ie combat shields and storm bolters and such, and you get death company guys for free depending on the units in your army. At the end of the day BA,BT,SW,DA are NOT standard codex armies so you cant use ANYTHING in the new codex it would be illegal. I know its a bit of a nightmare but that's the way it is. I feel sorry for crimson fists armies iam sure they had preferred enemy orks.
There is a new codex and new models. Check Gamesworkshop.com for more information.
Well Canis Wolfborn rides one and he's on a 60mm base.
AnswerGames workshop says it will release them soon after the new rule book.It will probably fall in an Inquisition codex combining the Ordo Malleus, Ordo Hereticus, and the Ordo Xenos. There are rules online approved by games workshop for a Deathwatch Kill Team. (the Ordo Xenos, or Alien Hunters) Rumors have come about that the Grey Knights are the next Space Marine Army to get revamped, so that could mean that an Inquisition Codex is comming out soon.
There is a rumor of a new race, nothing has been confirmed. Also there are rumors of either Tau, Necrons, or Dark Eldar are being re-released, and after that there's another rumor of a Space Marine army (probably Dark Angels or Black Templar) to be re-released.