These legs are part of a pant-system and attach to a pair of shorts.
They come in useful as an extra leg-layer when things get cold, are great for wearing early morning if there is a lot of dew on the undergrowth – so I don't get soaked, and also come in handy at camp at night to keep the mozzies from munching on my legs.
Both the shorts and legs I treat at home with TX Direct, I've been using this stuff since the 90's and it's a miracle potion that causes your garments to bead water and stay waterproof.
Shorts and pant legs available from REI for $64.
Theresa wears a similar set for women – I think the REI brand ones for $55.