Server Load Balancing products help direct traffic away from failed, overloaded, or simply far away sites and towards healthy, close sites with available excess capacity. Within the site, if multiple servers exist, the load can be shared among servers to provide further reliability and performance scalability. There are a lot of vendors with products in this space that vary widely in capability. These vendors vary quite a bit in the suite of features presently supported, but are all headed in the same basic direction. We don't know.
Nobody has everything it takes now.
Of the list above, Resonate and F5 Labs seem the furthest along that path. But it is entirely possible the eventual market leader won't be anyone from the short list above. Perhaps it will be a newcomer like IPivot or Arrowpoint, who generally have the right marketing messages and product directions as far as we can tell, but need to be shipping and proven in the field before they can make our short list. Possibly it will be Cisco, who as the biggest of the Big Boys also has the most sensitivity to systems issues, has credible products in the Server Load Balancing space today, and certainly has the resources and will to enhance them to become VRM systems. Maybe it will be a vendor who comes at the problem more from a content creation and management perspective, such as Atreve or Bright Tiger.