The Unreal Wiki is the biggest source for the Unreal Engine you will ever need. It has a huge database documenting nearly everything to do with Unreal, from how weapons work in the code, to importing a texture into UnrealEd. The Unreal Wiki is totally made by the community as well. This means, that you yourself can submit a page, or correct information that may be wrong in another page. This is great, and lets you submit your knowledge of Unreal to the many fans out there. Although the Unreal Wiki is a great source of knowledge, many of the pages of information dosen't exactly suit newbies to Unreal coding. There are a few tutorials there though, and they are farily detailed.
The design of the Unreal Wiki isn't all too cluttered, and is quite simple. It allows you to quickly get to the information your looking for. The search feature of the Unreal Wiki could be improved, as its a bugger to look for information fo a specific topic via this method. The community surrounding the Unreal Wiki are a really helpful source, and have pointed me in the right direction many times. This a great help, and if your new to Unreal coding, mapping, whatever, they will be able to send you to the right place and help you with your troubles. There is many things hidden around the Wiki too that you may find interesting. One interesting set of topics that may interest people transfering from one engine to the other is the "Unreal script for..." set. Basically put, these tell you what changes there are to a specific language to set it up for Unrealscript.
If you are new to coding, or an Unreal Engine veteran, you will find the resources that the Unreal Wiki provides of exceptional quality, and without a doubt, the best place for Unreal documentation on the net.