So when planing out allies for Tau, there really are not many holes to fill. Tau has plenty of access to melta (unlike say, Orks). It has a lot of mobility, ignore cover, monstrous creatures, flyers, etc.
The two main holes are lack of psychic defense (which is getting very scarce these days), and any real close combat punch. This is where Space Wolves really come in, as they have plenty of both to spare.
Now I'm going to concentrate on allies with Tau primary, so it limits the number of units available. Tau as the allied detachment is a completely different discussion.
Here is my first Wolf ally list, which I am currently working on painting:
The best part of Space Wolves is that they can take 2 HQ units per slot, even as an ally, which is in the FAQ. This allows both a rune priest for the "I say no" runic weapon and a powerful assault Thunderwolf Lord.
You also notice I pack a lot of melta in this list, with both Crisis Suits and Grey hunters maxed out. This isn't for vehicles as much as it is for Riptides and Wraithknights.
The commander goes with the Broadsides, to soak up wounds and make sure they always ignore cover so I'm not dependent on the Pathfinders. (I would normally take marker drones, but the pathfinders are cheaper and more flexible here, giving me more shots if needed) And if they do get assaulted, with the 2+ save and the invul they could survive more often then not and have a chance to get away since Hit and Run uses the highest initiative instead of the average.
The Rune Priest will usually go with the Long Fangs to doll out prescience. I will keep JoWW against Riptides or Necrons. I was thinking of some plasma cannons, but the S8 is a great help against the constant thorn that is Riptides/Wraithknights.
The thunderwolves will move up to take pressure off my gunline, but the best part of this list is the wolves don't have to go in alone, they can get support from Kroot and/or deepstriking suits. Saga of the Bear is a must-take, with even a Riptide able to instant-death him before he gets to swing. Dropping in 24 S5 shots will soften up any target, if need be. They probably won't deep strike too often, though.
I have a few more guys to assemble before I can start play testing, but I expect this list to do very well especially against the meta I face locally.
Now if you don't take the Thunderwolves (they do eat up a lot of points after all) then you can really go overboard on the shooting, going with 3 full units of 3 Crisis Suits all with 2 burst cannons. Not much is going to survive that. Add in more Grey Hunters, maybe one unit in a rhino or drop pod to go for an objective as well. I've seen this style on the table before, as one guy plays it, and it's pretty nasty.
That concludes part 1 of my Tau allies. I still have Orks and Bloo...*cough* Red Space Marines. I tried some Codex Marine allies before the new Tau dex came out, and it wasn't bad...running Assault Termies out of a Skies of Blood-ruled Stormraven. After that I'll dive into the TauDar uber combos.