If you can find one in decent shape, the old (like O.D.Green, pre-ripstop) Army ponchos worked great. They seem to hold up well too. The closest thing I've seen is the German Army version of the same poncho. The rip-stop ones are garbage, as are most commercial ones.
Of course the old heavy duty ones will keep the rain off, but you may die of dehydration from sweating under them, and you still wind up wet anyway, so there's that.