Well, I would mention The Lion King 2: Simba’s Pride and The Lion King 1½, but they were both already mentioned here. Yet, I’ll discuss them both anyway
The Lion King 2: Simba’s Pride:
Unlike quite a few people, I actually liked this one; it had a fairly good plotline, though the songs weren’t as good as the original’s (especially “One of Us”; it seemed out of place). It’s a shame that it’s not part of Disney’s Animated Canon (nor are all the other Direct-to-DVD sequels…).
The Lion King 1½:
Okay, I personally think this one was just as good as the original (though it’s not exactly a sequel; it’s a prequel/“interquel”). The plotline was great, and it explained quite a bit of information that the original left out (ie. Timon’s backstory). Not to mention, the songs were pretty good too. Again, this movie is not part of Disney’s Animated Canon…
Aside from those two movies, I don’t think I’ve seen too many of the Direct-to-DVD sequels (though I seem to recall that Aladdin had a couple sequels).