Well, solder without lead in it, obviously, since lead is toxic, meaning that if, say, you've used solder to join the pipes in your water supply, you'd rather it not be leaching lead into your system with every drink you take (especially since lead poisoning is a cumulative thing).

Lead-free plumbing solder is something like a 95:5 ratio of tin to antimony. When it first came out, it was a real pain to work with, having a higher melting point, narrower working temperature range, poorer flow and wetting characteristics, and just generally being annoying to work with. But the industry came up with some better formulations of solder and fluxes, such that lead-free soldering is now almost as pleasant as 50/50 (the old tin/lead plumbing solder) used to be.