It's possible to build SparkWeb without all of the tools we suggest (for example, by running the mxmlc compiler directly), but we haven't tried, so for the time being the requirements are as follows:
Flex Builder 3
Flash Player 9 (debug version, if you want debugging support)
To get and build the code, follow these steps
Install svn
Run svn checkout http://svn.igniterealtime.org/svn/repos/sparkweb/trunk SparkWeb
Open Flex Builder
Switch the workspace in Flex Builder (in the File menu on Macs) to the location you checked SparkWeb out to
Right click in the sidebar of Flex Builder and choose "import..."
Choose "Existing Projects into Workspace"
Locate the xiff directory in your sparkweb checkout, and select it
Repeat the previous three steps for SparkWeb, SparkWebCommonComponents, and SparkWebCore
You should be able to build and run SparkWeb now.