For those interested in illustration or specifically concept artwork, my personal favorite website for me is cghub.com. It's the cleanest website, where you can also find some of the best concept artists in the world in it. Each has their own profile with information regarding work, website, who they did work for etc etc. My favorite part of the website is that the first page has pieces of artwork that are some of the best/recent which filters out the rest of the work that isn't quite as good. I like this because sometimes you don't want to see any work, just come in quick for 5-10 minutes and look at the best work if you haven't been on the website for a week or more. It's convenient, and it works well in my opinion.
Obviously you can see early released work of everyone in the website in the specific section and even I myself cannot possibly compare myself to these greats yet (which is why I no longer put some of my work until I get better here unlike deviantArt). Not that I discourage anyone that visits the website to not put their work if it's not up to par with the best, I do this and my way sure as hell is not the best. It's just something I chose to do until I get a bit better and also I have not jumped into the concept artist genre specifically.

The website also has the typical things you need in places such as this. Challenges section where in the forums a challenge is to illustrate a certain object/monster/etc. and then whoever wishes creates it, then people vote for their favorites. It's a great way for fellow artists to get better, improve themselves by these challanges. I again, have not done this but I'm too busy nowadays unfortunately and have my own ideas that I have to flesh out first. I would recommend others to do this if they don't have anything to do and want to test themselves. The forums are well designed, clean and have a nice pop to it.
newest release section, 3D and 2D work next to each other
There are also lessons you can take from the website, obviously which you would have to pay. You learn from some of the best out there if people are interested. Another interesting side that they have is for those looking for jobs is that they have a solid amount of job postings out there for freelancers and potential full time work for those of you looking for it.

Just so if you are reading, you have a clear idea of what it looks like ^ and are too lazy to go to the website, I picked some images. One thing that did surprise me was how massive the amount of 3D work is at the website. There is plenty of 2D and 3D to go around for all that are interested. Some artists seem to be quite capable at both genres (those talented bastards...). Along with articles and blogs being added by artists, it has plenty of content, it's designed in an appealing way and overall works very well. For me, it's the best or rather my favorite concept art website though there's a few good ones out there which I will get to later on.