Most of the sources listed below have free applets. As freebies, they contain within the code or display the creators' mark in some way. Some also offer enhanced, professional versions for a shareware fee that removes the requirements for crediting and linking to the author, usually.

1Archive.com
6Sense.com
AnfyTeam
BuddySoft Java Applets
CodeBrain
DS Applets
Durius Java Programming
Dziners.com
FreeCode
FreewareJava and Tutorials
JavaComet.com - Home of the GhostDreamer Applets
HeHo Graphics and Java Experience
JARS - this site is more technical
JavaBoutique
Java City 2000
JavaPowered
RealApplets - Be aware of pay-for only applets here
WaterLogic Real Time Graphics - This is an interesting, artsy website and source.
Website Abstraction