I've been trying to find a lightweight head that can handle my 600mm lens and I can pack up in the mountains. The tripod and head required for those monster lenses go up in weight and price fast though.
With that said, the Gitzo G2380 head is small and light. It compares favorably to a Manfrotto 501 IMHO. It's too flexible for my setup, as it is one sided and not a bridge design, but for normal lenses it's "pretty" smooth.
The Manfrotto rep show me the new 502HDV head coming out this fall. I saw the pics of it at a local Outdoor Photo Expo associated with the Outdoor Retailer last week. Same design as the 504 and 509HDV's just smaller and lighter at about 3.5 lbs. It is intended to replace the 501. Looks pretty good on paper and might be worth a wait if you don't need something right away.
I use a 701 on my slider and sometime a crane with a HPX170 for a maximum load. It is a true fluid head, and is about the absolute minimum. Despite being small, it is pretty functional, just keep in mind what it is.