There are a few competitors to Outfitter: ItemRack, ClosetGnome, Wardrobe, and in 3.1, Blizzard's official outfit manager. I haven't played with the official version much but as you may have geussed, it's feature-light, but relatively easy to use and very stable, like Blizzard's official implementations of addons generally are (see: floating combat text, in-game calendar).
ItemRack has been around for ages, and I used to use it before I used Outfitter. It tries to display a bar onscreen that you can use to quickly swap through items for your various slots, but it's not hard to dissuade it from that outrageous waste of screen real estate (see right). It lacks Outfitter's facilities for automatically building outfits for things like maximum frost resistance, although such automatically-built outfits usually require extensive manual tweaking. ItemRack also lacks Outfitter's nifty "used by [outfit]" tooltips for items, but we're looking for Outfitter alternatives here, I guess. Edit: Yes, I should mention that ItemRack has lots of facilities for hotkeys, automated swapping, etc. It is the closest to Outfitter in terms of feature parity, for sure.
ClosetGnome is a minimalist item set manager. You can't edit sets, but if you save a set with the same name as an existing set, it overwrites. You can't automatically equip sets under certain conditions, although other mods can interface with ClosetGnome to cause this to happen for you. It is probably the lightest-weight outfit manager, in terms of resources, and it's probably what I'll use whenever Outfitter breaks, because I don't really need a more complete feature set.
Wardrobe-AL also exists. It seems to combine the small feature set of ClosetGnome with the hostile interface of ItemRack, but if the other two don't work for you for some reason, you could give this one a try.
As for the official outfit changer coming in 3.1, I just went and tried it out on the PTR, and the best I can say for it is that it works. It has the absolute minimum feature set possible: you can save a set of items, and load it later. You can't save a partial set (containing only certain slots), as you can with every mod listed above. (Edit: apparently you can by use of the Alt key -- is this ever told to the users?) You can't edit sets. Allegedly you can assign hotkeys to outfits, but I couldn't figure out how. You can't load outfits automatically on certain conditions (like stances or forms). It's nice to see this in the default UI, but it probably isn't going to cut it, at least not for me.
So, in conclusion: Use ItemRack if you need advanced features or for some reason like its abomination of an item switch frame, otherwise use ClosetGnome. If you can't stand to use addons, the Blizzard version will help you a bit (but then, if you can't stand to use addons, you're probably not reading this column). Let me know in the comments if I missed any good outfit mods. And to Mundocani: thanks for all your work. You will be missed in the addon community.