I would have to agree with Hymirl that Grey Knights and Imperial Guard both make for great allies, as they both shore up crucial weaknesses in our list. Grey Knights give us close combat ability (massed power weapons), durable units (t4 and 3+/2+ saves), and effective mid-strength guns (psycannons). Imperial Guard, meanwhile, give us the potential for cheap hordes of numbers, more and better tanks, and either more masses of flamers and meltas OR something we don't have- plasmas! Plus, as Battle Brothers, their Psykers can actually affect Sisters units, and while that isn't fluffy, the prospect of Celestine boosted with Iron Arm or Battle Sisters affected by Endurance is pretty nice.
Plus, of course, both groups give us flyers. And pretty damn good flyers, I might add.
Here is how I would field an allied contingents for both armies:
GREY KNIGHTS:
Ordo Malleus Inquisitor- Terminator armour, psycannon, nemesis daemonhammer, psyker, 3 servo-skulls- 119
5 Grey Knight Terminators- psycannon, daemonhammer, Justicar w. halberd- 225
Right off the bat, for just under 500 points, you have a unit that not only has nice firepower (especially with Prescience rerolls from the Inquisitor), but can also handle just about anything in close combat. The addition of servo-skulls means that you can also counter enemy Scout/Infiltrate shenanigans. To bulk up the allied force, you could add:
10 Inquisitorial Henchmen- 8 Psykers, 2 Warrior Acolytes- 88
-Rhino- 40
Vindicare Assassin- 145
The Henchmen (who don't take up a slot due to the weirdness of their entry) provide you with a big S10 AP1 template at 36" that, in conjunction with the Servo-Skulls, will probably hit its target (just don't perils, and definitely dont bring this unit if you know you're facing Eldar!) The Vindicare, meanwhile, can be used to pop off annoying enemy heavy weapons, squad leaders or even drop invul saves on enemy warlords, and he can potentially nail the AV14 vehicles that Exorcists will struggle with. The Vindicare is nice, but not a must, though, and is very pricey for a 2-wound model, so I would only take him at a higher-level game.
Oh yes, and if you want to give those Termies some mobility and dont want to deep strike them, consider the Stormraven. You could field it at its base cost and weapons, or add in things like multi-meltas or plasma cannons. Just be wary that, like everything with the Grey Knights, it can get pricey really fast. Again, unless you aim to make airborne Terminators a big part of your list, I would usually field the Stormraven at higher point games.
For Imperial Guard, meanwhile, I would take the following:
Primaris Psyker- 70
10 Veterans- 3 plasma guns- 115
Vendetta- 130
Again, a really good cluster of allies for under 500 points. The Vendetta-borned Veterans can swoop in mid-game, blow up a tank or flyer or two, and disembark to either hose down a tough target with plasmas, or grab an objective. The Primaris can join them, or ride with some Sisters. In either instance, I advocate Biomancy as the Primaris' power, as it has a good variety of witchfires, blessings and maledictions.
Alternately, of course, you can take a large blob of Guardsmen (30-50) with axe-wielding sergreants, stick Jacobus in there, and walk up and sit on objectives. I've seen Jacobus-enhanced Guard platoons in action before, and I can honestly say that such a squad will give your opponent fits.