Mindstorms.com is the official Lego Mindstorms site. Here you can download the free home version of the EV3 software, that latest updates to the EV3 firmware, building instructions for many more models that can be made with the home version of the EV3 and much more.
LegoMindstormsRobots.com is certainly worth a look.
Robot Square is an excellent robotics blog providing news and tutorials for building robots with LEGO MINDSTORMS EV3 and NXT, and occasionally the Raspberry Pi.
Legomindstormsev3.com is worth a look, though the site seems very slow.
Rato (Antwerp) seems to be the only Authorized Lego distributor in Belgium with an English language website. They sell EV3 education kits and, in addition to the standard sensors, some more exotic ones made by HiTechnic. These appear in the NXT section of the site but may be compatible with the EV3.
Dimitri (Dimi) is based in Belgium and has some interesting models with videos and (sometimes) building plans. I like DIZZ3 (pronounced Dizzy) which is a robot with only two wheels which does not balance like a Segway but instead has a center of gravity that is below the wheel axis. I'm sure you know what I mean ;>)
Dr. Graeme has all kinds of EV3 goodies, including tutorials.
www.bricklink.com is the unofficial Lego marketplace. 200 million visitors can't be wrong?
Electronic-Shop (Luxembourg) has decent prices on all things EV3.
HiTechnic manufactures a range of robotic sensors for the LEGO® MINDSTORMS® NXT and EV3. The majority of HiTechnic sensors are certified by The LEGO Company.
Robot Advance is a French company that supplies Mindstorms equipment in France, the UK, Belgium and elsewhere.
Robomatter Robotics Education Store features EV3 products developed by the Carnegie Mellon University Robotics Academy.
Mindsensors.com makes sensors for your EV3 or NXT robot. They also sell multiplexers and port splitters that let you attach more than one motor or sensor to a single EV3 port (too good to be true? Could this damage your EV3 brick?). And they sell relay drivers that allow your EV3 to control larger, non-Lego motors.
Génération Robots has some good EV3 pages. It's a French company but the website is in English, French and German.
NXT Video Trainer 2.0 DVD from Carnegie Mellon university. I suppose there will soon be an EV3 version.
Robotshop Europe has plenty of educational materials.
BrickPi lets you connect NXT and EV3 sensors and motors to a Raspberry Pi to make a robot and lets you power the whole thing with 6 AA batteries. One nice thing about this option is that you don't need the EV3 intelligent brick which accounts for two thirds of the cost a normal EV3 kit. See HERE and HERE.
I kept the best for last. The best EV3 site of all, is of course, Mind-storms.com.