There's a lot of digital versions, like the ones on Xbox Live Arcade and PSN.
There's also a site where you can play for free, it's http://www.playcatan.com/
Here's a review along with a description of the gameplay.
Here's a video of the version they made for Microsoft's Surface.
The game uses a modular board, which results in randomization of resource placement and rarity, among other things.
The basic idea is, you get to place some settlements and roads to start off with, and then everyone takes turns trying to build stuff. Everything is worth Victory Points, and the first person to 10 VP win. There are quite a few ways to get those 10 VP, and everyone has their own strategies for achieving whichever method they decide upon, so the gameplay can be quite varied.
The catch to this is, everything that you need to build requires resources, and there are 5 different types of resources (Lumber, Wool, Brick, Ore, Grain). Various resources are needed for each thing you build (a road requires 1 Lumber and 1 Brick, a city requires 2 Grain and 3 Ore, etc). You can acquire these resources yourself using your own resources (acquired from your initial settlements over time), or trade those resources to other players for something you want from them.
It's basically all about trading and building. The great thing is that, because it's played with 3-4 players, there is a very peculiar dynamic at work. Throughout the game, you are helping your opponents by trading with them, because it helps you in turn. When one person is about to win, the other players stop trading with them and will help each other gain points if it takes them away from the leader. People are cutthroat one minute, and going out of their way to help each other the very next.
As an example, the last game I won, I was about to lose at one point. I had 8 Victory Points, and another player had 9. He had everything he needed to build a city, which would put him at 10. In my hand, I had 3 Lumber and 3 Brick, but there was nothing I could do to prevent him from winning.
However, 2 of his Victory Points were from the "Longest Road," which you get for, as you might guess, having the longest road. His road was 9 long, and the second longest road was 7. My road was only 4 long. So what I did was, I traded all of my Lumber and Brick to the person with the second-longest road for nothing, and on his turn he (of course) extended his road 3 more, putting the road at 10.
He took the 2 Victory Points from the other player (he only had 5 to begin with), so when the other player built his city, it put him at 8 instead of 10. On my next turn, I built a city, putting me at the 10 I needed.
Looking at the situation, it's an interesting dynamic of cooperation. The other player helped me win, but it made the most sense for him because it stopped another player from winning. I gave all my cards to another player to help them get more points, and in the process I won the game because of it. Using strategy like this, you can take a losing situation and turn it in your favor.
The game's pretty easily accessible, as described at the top of the thread. It's available for download on Xbox Live Arcade and PlayStation Network. I know on Xbox Live Arcade, there is still an active player base, even though it's been out for quite a while. The game can also be played for free online at the site mentioned earlier.
If you have experience with this game, or tried it out, or have any questions, then you're welcome to reply.