well it REALLY depends on the tank size.
Dwarf sag is a really nice midground plant in my 5G :)
Personally, I like stem plants as midgrounds, because you can just trim them in half when they nearly reach the surface, and double the number of plants or sell/trade them.
I wouldn't reccomend dwarf sag as foreground, simply because it's perfect for a while, then suddenly shoots up to the top of the tank overnight.
Perhaps Pygmy chain sword
? (echinodorus tenellus)
In small tanks low-tech tanks you're pretty much limited to mosses.
I have stargrass as a foreground plant (planted it sideways) but once a week I have to bury the parts that start growing straight up.
It basically comes down to tall plants in the back, short plants in the front, and the ones that are in the middle, well, go in the middle.
Have to think about the max size though unless you want to move things around later.