Okay, chiming in.

XA by Dimension3D is a must for me now. Oh gosh, soooo handy. (Little toolbar where you can put stuff you use often, Content Paradise)

Shaderworks Library (available at RDNA) is also totally awesome and I wouldn't be without it now (Sorry D3D, I really didn't get on with XL)

I've also just bought the GI Light Studio over at RDNA (there's a sale on it atm) and I'm trying that out as we speak. Will report back on that one.
Another on my wishlist is the Light and Camera guide one from there, but it's a bit too expensive for me atm, so I have to wait for a sale. :)

Silke