linked_list.rb

class Node 
    attr_accessor :val,:nxt
    def initialize(val,nxt)
        @val = val
        @nxt = nxt
    end
end
def print_nodes(head) 
    while head do
        puts head.val
        head = head.nxt
    end
end
def reverse(head,prev) 
    if not head.nxt then
        head.nxt = prev
        return head
    end
    cnxt = head.nxt
    head.nxt = prev
    return reverse(cnxt,head)
end
first = Node.new('1',Node.new('2',Node.new('3',Node.new('4',nil))))
first = reverse(first,nil)
print_nodes(first)

Or, the obligatory code golf:

def reverse(head,prev) 
    head.nxt = prev and return head if not head.nxt
    cnxt = head.nxt and head.nxt = prev
    return reverse(cnxt,head)
end