You can find most of the shareware and freeware AppleScript scripting additions at www.osaxen.com.
You can download AppleScript-related software at www.macupdate.com or www.versiontracker.com.
For ready-to-run and open-source scripts, you can always find great scripts here too, in our own ScriptBuilders allways growing archive, which has actually up to 1200 scripts divided in 21 categories!
Also, you can snag lots of code at Code Exchange, our special section to share ideas and experiments.