There are a great deal of wonderful go resources on the web. Here are a handful that members of the Pittsburgh Go Association often reccomend.
- KGS - The most popular go server in the western hemisphere
- Sensei's Library - Wikipedia of Go
- GoGameGuru - Great resource for news and problems (they also have English subtitled Baduk TV for purchase)
Mobile AppsFor Android, I like Tsumego Pro and the WBaduk app, which are both free. But there are many more. And for iPhone/iPad there are even better options, especially if you are willing to pay. You should definitely check them out; go apps are a great way to casually improve.