Do note that if an addon doesn't have a specific license attached, due to the
Berne Convention, that addon's license is "by default" All Rights Reserved.
What that actually means for addons isn't exactly easy to nail down, due to the interaction of Blizzard's guidelines and RR licensing. Here's another thread where some people discuss it:
http://www.wowinterface.com/forums/s...ad.php?t=48570
You can be sure that it means, at least, that you cannot literally copy the addon in its entirety or any of its parts and paste them into yours. That is actually and clearly illegal. Most authors, as several expressed in that thread, are okay with you reading through their code and learning from it -- even copying the
spirit of novel techniques they may have used to solve various problems.