I've found 2 cups of high quality neem cake added to a 5 gallon bucket and then bubbled or stirred for a day or two to solubilize will work well in killing fungus gnat larvae. I've also used it to slow down root aphids (it didn't eradicate them though). There was concern that the neem cake my kill off beneficial microorganisms but my experience under the microscope showed it actually increased aerobic bacteria.
Also, here's my favorite quick read on neem:
http://www.agrineem.com/PDF-Files/FUNCTIONAL BLOCKS OF NEEM OIL.pdf