The Unreal Development Kit can be downloaded from here. If you want to develop your own content for it however you will need to have about 8GB of RAM installed.
You can get FlashDevelop the free IDE from here. You should also take a look at the Wiki here for other useful information & the Flex 3 SDK can be downloaded from here.
More information can be found in this thread on the DBF forum.
Is a platform for building & hosting Web applications which run on Google Servers. I've just downloaded the App Engine SDK from here. You'll find more about the 'Google App Engine' in this Wikipedia article & there is a simple installation guide & 'Hello, World!' program here.
This is a video which shows you what the 'App Engine' is all about.