For a single loop I either use i or x. I use x fairly often, especially when iterating an array, because I often think about it as if I am iterating over the "x-axis". Consequently, with matrixes, I often use x and y. I guess this comes from doing a lot of work with graphics. z is also used if it is nested another time.
Other than that I use i, j, and k fairly often as well. I usually kind of alternate between x,y,z and i,j,k.
When a LOT of loops are in order...I will go off to l, n, p, a, b, and c.
With iterators, usually something like iter, iter1, iter2, or maybe myIter, myIter1, myIter2. I tend to put the prefix of "my" on a lot of variables...it is a thing that has rubbed off on me since high school when we would work a lot with the "AP classes" for strings, vectors, and matrixes...and they named their variables like that.