I am intrigued by nematophagous fungi, though haven't used them. There was a mushroom company (Fungi Perfecti) that had spores for sale that you'd work into your garden. (Can't seem to find the site now.) This type of fungi actively kills nematodes. Here's a cool video on how they do so:
http://www.youtube.com/watch?v=0n04wCkIpuQ