Wow, that's an interesting question!
We use all kinds of stuff, depending on whether you want to build gaming or display bases / dioramas.
Birdsand and static grass is the first thing that comes to my mind when you want to build standard gaming bases.
Wood glue mixed with the corresponding paint if you want to create lava bases or, for example, green slime bases for a nurgle army.
All-purpose glue for spiderwebs.
Cotton wool balls to create smoke or when you use them with wood glue, you could create climbing plants.
Or you could go for a walk and collect twigs if you want to create trees on your bases.
Or go to a building supplies store and buy impact sound insulation. Their reverse side sometimes looks like wood planks.
I use cork to create rocks and paint them grey.
Greenstuff World offers textured rolling pins for all kinds of bases. You can use them with Milliput or Greenstuff to create cobblestone pavement, steampunk bases, temple bases, wood planks or bases with celtic runes.
There is water effect to create water bases.
Tiny animals, leaves or paper plants you can add to your bases.
The usual GW skulls, of course.
And if you have a 3D printer or use resin, there are endless possibilities you can print to embellish your bases: boxes, mushrooms, tree stumps, crystals ...
Crackle medium for desert bases.
Fly-screen or toothpicks to create fences or spikes.