I've been working on some stuff about prime quadruplets. For reasons too dull to explain just now I started by looking for a formula that always generates composite numbers. At some point when I get the chance I'll explain this a bit more, but after a bit of thought I came up with this:
2x -1+4y(x+y) for all positive integer values of x and y.
It does work, which is nice - it generates all the odd composites (which is all you need since all evens are composite except 2.
Edit: Obviously a formula like xy where x and y are positive integers > 1 is a great deal simpler, but for various reasons I wanted one for which you didn't need to exclude 1.