There are two big issues for those of us who have to lug our own stuff: weight and bulk.
The obvious answer to both these issues is to go to the smallest possible scales, but sometimes the appeal of a game lies specifically in using larger figures, i.e. 25/28mm, and then scenery can become a significant challenge.
There are a few ways to get round this, e.g. don't try to do Stalingrad in 28mm, but this post is concerned specifically in paying tribute to a couple of manufacturers whose products are particularly lightweight.
First up are the hollow resin hills from Tiger Terrain which are both very light and very strong. The company originally did a range of these - green, arid and unfinished. They still have some advertised on their website but I think they're being phased out. This is a pity. I'm not aware of anyone else making anything like these.
The other company I'd like to praise is TableScape. As I understand it, their products are also made from resin but they add a foaming agent so that their products emerge from the process as a strong, dense foam which is both light and robust.
I understand this process is unsuited to small items, so their terrain is restricted to 28mm products. I have a lot of their Renaissance buildings and have recently started to accumulate their Islamic-style pieces. The buildings come ready-painted and are extremely high quality.