Cab-o-sil is the same thing as colidial silica. Available at CST, aerospace composites, and I'm sure many other places.

I like to use it when filling small gaps at the base of the V-tail. It helps eliminate the possibility of the epoxy running down into the fuse and mucking up your linkage.

Super-fil is a lightweight two part epoxy paste that makes great fillets. Its very light, and very easy to sand. Only downside is that it takes quite a while to dry. I leave it overnight here. Its available at Aircraft Spruce.