If you're new to blender, still developing your skills or even are an expert at it the Blenderartists Weekend Challenge is a great way to hone your skills. I usually check back there every week or two to see what the theme of the contests is and if I like it and have the time then I'll enter. You can read up on the rules here and then you can start designing!
For last weekend's Weekend Challenge the theme was "Cityscape" so after some initial concept creation I made this:
I decided to make this a big learning experience as well so I split the scene up into layers and composited all the render layers together. There are 3 separate layers: the foreground (The buildings in the front and some of the water), the background (all the far off water and buildings) and the sky (moon, stars, etc.). I then applied some colouring effects (making everything a little blue) as well as giving some bloom and a little background bluring. I modeled all the buildings, platforms and bridges, then I lit it all (extremely hard work I must say.) and got everything the way I wanted it to be.
My final result was what you see below:
I used the .blend files from a Blender Guru tutorial to give me some hints on how to create my highrises, and from there I was on my own.
I do wish I had gotten it a bit more complete, maybe added some people walking around, some more building variation and I was hoping to create some sort of transportation system (monorail or something.) however I am very happy with what I accomplished and learned in my latest foray into the world of 3D.