SwfScan is a simple free SWF decompiler from HP's Web Security Research Group.
Just open your chosen applet and it'll immediately be decompiled, allowing you to browse its code. If this is lengthy, then a search tool (with regular expression support) will help you to locate the details you need. Or you can export the code to a text file for analysis later.
SwfScan can also uncover a few security vulnerabilities in your source code. Just click the Analyze button, then select an option in the Vulnerabilities list for an explanation of the problem.
And the program also scans for URL's within the code, which may be handy if (for instance) an applet is streaming a video and you'd like to download a local copy for yourself.
Please note, though, SwfScan doesn't offer any form of resource extraction. Your target applet may have videos, images, icons and audio files built in, but SwfScan won't allow you to remove these.