How about Paladin,Bonanza, Gunsmoke, The Wild Wild West, The Lone Ranger and Wagon Train.
@nature700 Welcome Back Kotter was before my time but I remember loving it back when it was on Nick at Night.
For me there are shows on Nickelodeon that I miss that never completely made it to VHS/DVD or streaming. The two I miss the most are Salute Your Shorts and The Adventures of Pete & Pete. They have some episodes online, but not the entire catalog, which is such a bummer.
Has Cheers been mentioned? I miss Norm and Cliff.