Some of the other campaigns that I can think of are Procyon Insurgency and the Ancient Shivan War. Both are very well done, although PI can be rather challenging, and ASW only has the first of three chapters released.
Yes, there is a LOT to download, and if you are one of those poor souls stuck with dial-up (like me up until a few months ago, when I finally got DSL), it is virtually impossible to get all of downloads for FSO and the mediavps, much less any of the mods. I found Hard Light about a year ago, but never joined because I couldn't hope to actually download everything, but finally got DSL and basically went on a Freespace playing spree, and the amount of work this community has put into the various mods is truly amazing.