I am wondering what possible uses there are for these things. Will they carry the weight of a car over a small ditch? My friend and I are thinking of purchasing some to take offroading- to add weight to the vehicle for a little traction and to get us out of crappy situations. Im curious if they can be used for anything else- offroading or around the house or for an emergency. Any ideas?

just in case you don't know what it is, it is the steel mats you see in the picture below

Wow where are you going to get them?  I've not ever seen any for sale.

I've seen the mats made into jeep trailers using scrap oilfield tubing for the frame and a recycled mobile home axle.  The mats just made the bed of the trailer.

I've seen them used as trailer ramps for a bobcat loader.  In this application they bent.  about every third use the mat had to be driven over with a concrete mixer truck to straighten them.

The mat really needs to have ground under it.  They should keep soil from shifting out from under a vehicle but they really can't hold up a vehicle on their own.  They aint a bridge.

the mats are strong enough for planes to land on

I found them locally for $35 a piece. The seller has some listed on EBAY for the same price but shipping is like $25 (a steal for a 60 lb piece of metal). I also was thinking that since they were used for aircraft I could drive over them, but my father (a former Seabee) says it is supposed to have ground underneath it. I like the idea of making a trailer out of it. Any other ideas?

At $35 each I think you could make a better investment by buying something else.  To bridge a ditch consider a portable ramp like those sold to get garden tractors into pickups.  As far as for a trailer FREE is one thing but if I spend money on it I want it to look professionally presentable in public.

Not sure how stiff the Marston Mats are - but doubt they can be used as Bridging Ladders. Likely good enough for Sand Ladders - and thats not a bad price compared to whats commerically available - which are often fiberglass/polymer grids or something close enough to Marston Mats.

The Fiber mats can be used as bridging mats - but notice the deflection



Sand Ladder




This is a Bridging Ladder




If you need, you can make a a fairly durable log bridge fairly easy





Luck
Alac