Best Friends Forever!
Salamanders, Iron Hands, Space Wolves, Dark Angels, Tau, Imperial Guard, Eldar, Oh MY!  

I am sure that we can all agree that White Scars Chapter Tactics are pretty awesome if you are fielding an army solely comprised of bikes.  However, what happens if you want to try out other play styles or maximize your army's capability?  This is where allies come in.  They can take weaknesses that your list may have and correct them in order to have a more well rounded list. 
So here comes the good news and the bad news... there are way to many effective ally combinations.  Every single combo adds something to the army and can radically change how the list is played.


Salamanders

Master crafted combi weapons anyone?
Deep Striking element
Twin Linked flamers
Assault Marines and Land Speeder Storms really shine with Salamanders

Salamanders are probably best if you are looking to maximize flamer and meltas in your build.  The best way to do so is with drop pods.  A 5 man assault squad with 2x flamers and a combi flamer in a drop pod is only 105 points of awesome.   Therefore, if you are looking for a more aggressive style of play and the maximize alpha strike potential, I think Salamanders is a great way to do that. 

Iron Hands

It will not die Chapter masters
It will not die on vehicles
Feel no pain (6 +) to make units more durable

I heard you like character hammer?  A chapter master with an eternal shield and a 6+ feel no pain riding around with your bike squads can be an absolute pain to deal with, especially if they are consistently regaining wounds.   6+ feel no pain also helps out fragile scouts to make them more effective at securing objectives.  Also, if you are taking a chapter master, 6+ feel no pain goes a long way with honor guard. IWND on vehicles definitely helps,  especially on land raiders, but I doubt the effectiveness of keeping storm talons alive. 

Space Wolves

The best Psychic Defense in the game
Jaws of the Wolf 
Access to Divination
Chooser of the Slain
Better Devastators
Deep Striking element
Wolfguard with combi weapons
Grey Hunters

Usually when I am pitching bike oriented lists to my friends, the first suggestion that comes up is Space Wolves. Rune priests are just awesome and the fact you can have 2 of them for 1 hq slot is even better.  Combi weapon toting wolfguard and 5 missile launcher long fangs are awesome as well.   My problem with them as allies is 2 fold.  1) They will get a new codex sooner or later and 2) It is expensive to get the most out of them. I am not saying it can't be done, but in all of my list building attempts, the lists appear to be more Space Wolves then White Scars.

Dark Angels
Access to Divination
Land Speeder Shroud
Power Field Generator
Rad Grenades
Plasma Talons
Move Through Cover

Having trouble keeping your bikes alive?  Well Dark Angels can help out a lot with that!  A librarian with auspex and power field generator (4+ inv save for models within 3 inches of the libby) can massively boost the effectiveness and sustainability of the grav command squad.  Did I mention that the libby now gives access to a plasma toting black knight squad?  That unit, with rad grenades will win games.  To wrap it all together, squeeze in a land speeder shroud to add to everyone's cover save.  2+ Cover on TFCs?  A possibility.  3+ cover for moving a bike? Can be done.  Sure certain armies just annihilate the shroud, but against those that can't it is going to be worth while. 

Tau

Cheap Scoring units
Commander Helpful
Supporting fire
Broadsides and Riptides
Sniper Drones

For whatever reason, my grav gun command squad seems to miss more then hit when shooting at full capacity.  Since tau and space marines are battle brothers, I can attach a tau commander to that unit to not only make them twin linked, but ignore cover while having the choice of monster hunter, tank hunter, counter attack, and stubborn.  The tau commander can also be a viable source of tanking wounds with his iridium armor.  Tau allies also add a substantial amount of long range, high quantity, fire power potential.  Broadsides in fortified terrain thanks to a techmarine?  Completely possible.  Personally, Tau is probably best for a defensively oriented style of play. 

Imperial Guard

Bodies... Lots of bodies
Hydras and Vendettas
Manticores
Loads of special weapons

In the vast majority of the games that I play, I am often left with a handful of models.  The problem is that a bike army simply doesn't have that many models on the table, let alone scoring ones.  Imperial Guard platoons can dramatically change that.  Looking to "bring your own terrain"?  Well a bunch of chimeras and leman russes not only block LoS to bikes but are cheap and dish out a decent amount of fire power.  Looking for even more special weapons, veteran squads can tote 3 plasma guns and a lascannon for next to nothing.  Also, lets not forget that they have some of the best anti-air in the game. 


In short, there are numerous possibilities for allies that are all worth considering.  This is only a small section and I did not even dive into allies of convenience, other space marine chapters, or blood angels.  So what are your current ally combos for White Scars looking like?