There is a new Serious Sam 3 update on Steam which came out today. Long story short: 7 new versus map, 3 new survivor map, VAC protection and lots of bug fixes.
The new DLC for Serious Sam 3, aka Jewel of the Nile, coming out tomorrow.
The day after tomorrow is the release day of Serious Sam 3 on xbox 360. Singleplayer and multiplyer will be separated. Trailer
Unfortunately, the release date of Serious Sam Double D XXL is unknown. It might be coming out during the winter holidays.

(UPDATE: New DLC trailer and to celebrate the xbox release have some kamikaze ringtones)

A new DLC for Serious Sam 3 called Jewel of the Nile has been announced. The story takes place in the middle of Serious Sam 3 storyline. Three campaign levels, new enemies and also new weapons. The DLC comes out in October.

The multiplayer of Serious Sam HD The Second Encounter from now free-to-play. More specifically the Capture the Flag, Deathmatch, Team Deathmatch, Instant Kill, Last Man Standing, My Burden, Survival and Team Survival modes. Also, a new DLC called Legend of the Beast is available, with three new campaign, three new versus, and a survivor map. Source

Other small news: You can play Serious Sam 3 on Macs, and Serious Sam Double D XXL is coming on Xbox 360 which is an extended version of SSDD with lots of new features, hopefully co-op multiplayer.
(UPDATE: if you want to play with the F2P version of SSHDTSE click on download demo button. Also, SS3 and SSDDXXL coming this autumn.

You can buy every Serious Sam game (at least those ones which are on Steam) in one pack here. Also each one of them are 66% off this weekend.
And finally some Serious Sam history here

Serious Sam 2 is now available on Steam too.

You can buy a new Serious Sam indie royale pack, which contains Serious Sam Double D, Serious Sam The Random Encounter, Serious Sam Kamikaze Attack, Serious Sam The First Encounter, Serious Sam The Second Encounter and a brand new one: Serious Sam The Greek Encounter. If you want to buy it hurry up, it's only available for a few hours. The price is constantly changing.

Some Serious Sam 3 news (Spoiler alert):
UNDERCODE - Final Fight | Fork Parker Trailer | Rich Knuckles - Blood options | Rich Knuckles - Color options
RockPaperShotgun review | NeoSeeker review | Destructoid review (Video) | DIYGamer review | CVG review | 1up review | Incgamers review | Slipgate Central review | The Escapist review | Arstechnic areview
Also, the new update to Serious Sam 3 brings free Santa Sam and Santa Sammy character skins.

Serious Sam 3 is finally available. Launch Trailer: here

Another trailer. Starring: Blood.
Trailer

The trailer shows the melee attacks in Serious Sam 3.
Trailer