In-line ones for external filters, some externals come with heaters inside them and undergravel (not really any good at heating the water itself).
Out of the in-tank heaters (the tubes), there are the usual glass ones and also polymer ones.
I prefer to use in-tank heaters because if my filter breaks, I don't want my fish to get cold. If you're worried about looks, the NeWatt polymer heaters are almost not visible on a black background.