Under modern copyright law, you do not need to do anything special to copyright something you create. The moment you create it, it is automatically copyright in most civilised countries, and for many of them you don't even need to say that it's copyright. While it is possible to register your copyright with a government, this is not a necessary step; it's just a way to make it a bit easier to sue later.
So, the copyrights on plug-ins you download most likely are valid, and when you create a plug-in, all original content in it is automatically copyright to you, although you should probably add a copyright message just for safety's sake. Obviously, content that you didn't create or pay for the creation of (ie. works for hire) remains copyrighted by its creator even if you include it in your plug-in.
On the other hand, if you create a plug-in and want it to ~not~ be copyright, you can do that by placing a note in the readme file saying that you have placed it in the public domain.
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David Arthur
(url="http://"http://members.aol.com/darthur1/talon-ev/")Talon Plugin for the original Escape Velocity(/url)
(url="http://"http://members.aol.com/darthur1/mc/")An Upcoming EV Override Mission Editor(/url)
(This message has been edited by David Arthur (edited 12-09-2001).)