I researched to find differences between the solder alloys I have used in the past to solder various things (tin/lead 60/40) and have found that lead-free is harder to melt and is grainy when cooled, and as such cannot think of why someone would want to use it over tin/lead or some other combination.
I considered the fact that people may worry about (their health?) lead in the solder, but I'm fairly sure this point is moot if you are soldering properly with proper gear.
So please, tell me what solder you use in your builds, and why.
Alternatively/Additionally, take this strawpoll for a more analytical approach as to who uses what:
Edit: fixed a grammatical error