Singed is one of my favorite heroes, if not my favorite, and I've figured out an early-to-mid game item build that I'm fond of. First regen pendant, then boots, then mana/health stones leading into Rod of Ages (sometimes I'll get a Catalyst before my boots if some extra tankability is needed), and then, depending on the enemy's setup, either a Force of Nature or a Warmog's Armor. After that, I usually just end up falling back on the reccomendeds.
But, there are many players more experienced than I. So I ask you, Singed players, what do you usually plunk down on late in the game?