Most of the stuff I can recommend is commercial software...
Animated GIF construction hasn't hit open source yet (the GIF patent expired just recently (2003), which was the original hold-up) -- and now that it's expired, it doesn't matter, because GIF sucks and is an archaic format with no extensibility.
See also
http://www.gnu.org/philosophy/gif.html for a tip of the iceberg of reasons that GIF is evil.
also
http://burnallgifs.org/http://www.kuro5hin.org/story/2003/6/19/35919/4079MNG is the preferred format, as it is open (as its brother PNG) and is based on the vastly superior PNG format.
As for programs, I still can't recommend anything, because again, most animation studio programs are commercial. I'd have to recommend Adobe Imageready and Paint Shop Pro's Animation Studio, on the commercial side.