Animation Shop 3. Its advantage was the ability to work on your Photoshop layers as frames and just save the psd, as Animation Shop would allow to load it and make a frame of each automatically. That together with a great onion skin setting (ability to preview previous and/or next frames in transparency, to do a good animation) made it cool for sprites animation. These days though, Gimp can open a psd with not many layer effects on it (many softwares have problems with that, as it's native Photoshop stuff) . Animation Shop would also allow some filters and transitions, you had a nice collection there. It had decent tools for optimizing the gif (pallete optimization and a large etc) you would output (these days, if I am forced to add an animated gif, I try to make it not bigger than 200ks). For games, one could export rgb frames, and even create "tubes", sort of filmstrips with all frames together in a row, in a single graphic.
Probably is still recommended for people not liking much Gimp (I like Gimp) , or Photoshop Animation Window and tools (and/or Image Ready for that matter). Despite says in their site it wont work on Vista, I tested that at least 3.05 from Softpedia worked perfectly in Windows 7. So, don't blame me if it does not work after purchasing.(a last resource is registering the Softpedia eval version, maybe.) You can get it at Corel for 19 bucks.
Personally, these days as I work more on design than game art, I rather prefer Gimp (ie, using Gimp blend feature, etc) or Photoshop (it has built in a tween ability that allow fades, etc, though is tricky), due to their editing capabilities.