So, I've been talking to a few people recently over a potentially awesome new list you can field with the new Grey Knights codex. I'll start out with the basics and we'll see how long it takes to catch on to the plan.
1. Servo Skulls. Several HQs and an Elite choice can take 3 each for some minimal cost like 5 points a piece. They reduce scatter on Deepstrikes and Blast Weapons as well as limiting Scouts/Infiltrate moves within 12". They can be fielded anywhere outside the enemy deployment zone. When the enemy gets within 6" they die.
2. Orbital Strike Relay (OSR). Amazing in the new codex. Three choices: Barrage Bomb, Lance Strike, Psyk-out Bomb. UNLIMITED uses. You can fire one per turn during Shooting provided you did not move during Movement. So you can have one model firing up to 7 Orbital Strikes. You do need LOS, but the range is Unlimited!
3. Grand Masters and Brother-Captains can take OSRs. Both your HQs can have them. Awesome. For some variation, Inquisitor Lord Karamazov is particularly mean. He can use his own men as "Orbital Strike Beacons." He costs the same as a Brother-Captain with OSR.
4. Techmarines. Techmarines? Whiskey Tango Foxtrot, sir. Yes. You know why? 140 point Orbital Strikes. Every Techmarine can take an OSR for an upgrade, so you can bolster some ruins and chill in them, calling down strikes. Why yes, they are the Elite choice that can take Servo Skulls.
5. Warp Quake. Strike Squads can prevent deepstriking. Ld 8 or 9 Psyker check.
Therefore I present EXTERMINATUS:
2x Brother-Captain with OSR, 3 Servo Skulls
3x Techmarine with OSR, 3 Servo Skulls
4x 10-man Grey Knight Strike Squad
Of course, I don't have the codex yet, so I don't have the exact costs, but that should fall into the 1700-1750 range depending on the upgrades chosen. Basically you take your 15 skulls and grid them out over the field. You will lose a lot to enemy movement. They're just Servo Skulls. Who cares? The point is that if they destroy one, they should be in 12" range of another. Your OSR bearers will sit back and wait for the enemy to come to you. And to stymie those efforts, you have Strike Squads to prevent Deepstriking and Skulls to prevent Scout and Infiltrates. If the enemy does make it across the board, they have to make it through Strike Squads.
Another fun trick is to spend another 25 points to make a Brother-Captain a Grand Master so you can make units Scoring. This could be your Techmarines, or it could be Interceptors if you give up a Strike Squad (or if playing sufficiently high point games not even that).
The End Result is EXTERMINATUS: Up to 35 Orbital Strikes in one game. It's probably not the most competitive list, but it's gimmicky and silly enough that it'll probably throw your opponent off-kilter. For kicks, add in a house-rule: After 6 Orbital Strikes in one game, the entire board takes a S10 AP1 hit. Obviously the armies got caught in an EXTERMINATUS attempt during the battle.