Mods like Armamentatirum and FCOM add a ton of new armors to the game. Notice how I said armor, not robes. There are some robes added by these mods, but they just aren't at the same level as some of the unique armors from those mods. So, where is the love for mages? I want to play a mage without an armor skill, but then I feel like I am missing out on a ton of really nice content (All those armors)! What are your favorite mods that add unique robes to the game?

Do a search for robes on Nexus. You'll find quite a few.

gothemasticator

Try http://www.tesnexus.com/downloads/file.php?id=12996, which lets you (among other things) convert armor into clothing.

One of my favorite mods! My mage uses it all the time.

Tegeal's Robe Pack, Slof's Oblivion Robe Trader, Blackheart and Gizmodian's Robes, Tona's Simple Dress Armor and Anchorite outfits- my female mage wears a lot of stuff from these mods. There are male versions of all these as well. Try an advanced search for anything with Robert in the title if you're looking for stuff for guys.

Hard to go wrong with http://www.tesnexus.com/downloads/file.php?id=24320

I generally play a light Armor guy and use it just because NPC casters look so much amazingly better.

The thing with robes is.... they usually work better as quest rewards. With armor, they're just armor so I could see them being bought/sold, but with robes they're either simple impractical* clothing, or heavily enchanted. I haven't tried many robe mods, but I can recommend some quest mods that have robe rewards(all of which are enchanted):

Integration (3, or more robes and 1 or more fineries(sp?) )
Servant of the Dawn (2 or more robes)

Gotta go now, might think of more later, and if you're looking for new meshes/textures you might be better off with the robe mods since most of the quest reward robes I can think of use vanilla resources.

*They offer no protection, and take up two or more clothing slots. So practically speaking, you'd be better off custom enchanting a shirt/pants combo as opposed to robes.

Robes occupy multiple slots, so they don't really work unless they come pre-enchanted with powers proportionate to the number of slots they use. Or if you have a mod that makes enchantments on multislot clothing more powerfull than the same enchantments on single-slot clothing.

Also, they don't really have all that much to do with mages. Lots of mages wear armor or non-robe clothing. Heck, in vanilla Oblivion rapid casting requires a shield, at which point there's little or no additional casting efficiency penalty for armor if you have a decent armor skill.

Using Supreme Magicka, LAME, and RBP gives a pretty big penalty to armor-wearing mages though

I've always worn light armor, and it's never imposed much of a casting penalty.