So the Teleport Hub site threw up a pretty complete calendar listing, yay for them. Feel free to go down that list on its own; these were pretty much the ones that seemed useful for me, with either free-to-join groups, or no groups needed.
And I'll be adding to this list as I go, so I'll post the link in future entries to check back. Also, the Showtime blog has an Advent list up.
[Late edit: Found another list containing some different Advent links from the Chica and Phil's Place blog. Also, the FabFree blog has a list of Advent stores.]
Links that require no group:
A Touch of Magic :: Holiday decor.
Caledon Oxbridge Rose Garden :: Could be anything, really, but I'd largely assume bits of neo-Victorian and steampunk decor, Caledonian textures, steam pets, et cetera.
Chaos, Panic and Disorder :: Will likely be home decor items, accessories, and the like.
Ch's :: Seems to be mostly clothing and avatar toys. Gives out past days' gifts, as well.
C.M.D. Art and Furniture (rated ADULT) :: Likely home decor, some items may have Adult poses.
Dulce Secrets :: Eyes and avatar attachments, I think.
Eclectic Stars: Geeky accessories and clothing.
Emerald Couture :: Clothing.
Fair Play: The Forgotten Game :: Various fantasy/gothic themed items; track down the glowing dates, click on that date, get the gift.
Fairy Unique Designs :: Textures. (Pay L$1 to get the gift, it will be refunded.)
FallnAngel Creations :: Clothing, hair, or skins, if the last few years are any indication. Also check out the Holiday Lucky Presents tree.
Formanails :: Nails and nail accessories.
Fuzzy Peach :: Likely home decor. Can still get previous days' gifts as well.
Grumble :: Likely clothing. (I have no idea what she's giving out, I'm still banned from the sim.)
Helping Haven :: Holiday decor and system-layer clothing.
Isis Boutique :: Will mostly be shoes, gifts go from L$1 to L$24.
KittyCatS :: Because this is a donation calendar every year, it can be literally ANYTHING--so expect clothing, accessories and jewelry, shoes, decor, holiday things, as well as pets.
Lantian Flox :: Home decor items, likely holiday-themed.
Needful Pixel (rated ADULT) :: Think mostly clothing and shoes with a fetishy bent.
Ollie's Follies :: I'd assume home decor. (Sit on Santa's lap, get random gift.)
Perfect Seduction :: Clothing and accessories.
Skye Qi :: If previous years are any indication, the entire Advent contains pieces of one or two outfits. If you miss a day, just buy the day previous--they're priced according to the date.
Timeless Textures :: Textures.
Links that require (free) groups:
7 Deadly Skins :: Generally skins and outfits, occasionally jewelry. (Group is free until she hits 30,000 group members, so keep that in mind.)
CIRCA Living :: Home decor items, varying in cost (between L$1 and L$75).
Entice :: Stockings, clothing, accessories? Could be anything. (Pay L$1 to get the gift, it will be refunded.)
Freebiecosmos :: Could be anything from clothing to holiday items to furniture.
Hello Dave :: Nails/appliers.
Inkheart :: Will likely be eyes. Also scan around while you're there for hunt items, because generally all of their hunt items? Also eyes.
Loordes of London :: Shoes, outfits, for the most part. (Calendar seems to be reversed; wear group tag and hit the end-of-month days first.)
Mariposa Gardens :: Home and garden decor, possibly holiday-themed.
Roots and Wings Clothing :: Mesh clothing items, some may be holiday-themed.
Yasum Designs :: Seems to be accessories, at least to start.
That's all I've got for now! Have at!