sParticular, by Sebastien Perier, is a script that helps you with 3D integration when using Trapcode Particular. Running sParticular will create as many Particular instances as needed between all your 3D layer and spread the particles in your 3D world, all linked to a master instance you can edit if you need to tweak your particles. Be sure to also check out the sParticular homeland.
The Immigration script "turns the pain usually associated with importing and batch replacing files in After Effects a joy. While browsing for import, it show file sequences to one line item instead of a long list of files. Immigration uses a custom algorithm to scan the file system at speeds dramatically faster than even the OS can. In our testing a server folder with 30,000 files that takes over 5 minutes to open in the finder opens in 12 seconds in Immigration (and that includes file-sequence detection!)." There's more info on additional features at AEScripts.
Immigratation is the first commercial script for After Effects, and there's a $10 off promotion; just enter the promo code ImmigrationLaunch at the checkout for for the discount.Update: There was a nice clarification on sParticular on the AE-List. Phil Spitler asked: "This sounds great, but by putting the 2d Particular layers between the 3d layers, doesn't it break the render order between the 3d layers?"
Sebastian answered: "Yes it does sadly. So if you have intersecting 3D layers, it will break their intersection. But if not, as long as your layers are properly sorted in Z space, you shouldn't have problems.
It's more a workaround than a definitive answer sadly. But still, you can find some workarounds like rendering your 3D world without the particules (or only without the instance that cause intersection problems), and then render the particules alone using your 3D layers as track matte and then composite your 2 renders together. But yes, true 3D intersection will still be a problem."