For me, it's LinkedIn. Now, I know it's been increasing in popularity over the past few years and during the past 15 months it has literally been flooded with salespeople who never thought much of it in times past. BUT, out of all the tools I've used over the past 10 years or so, it consistently allows for me to engage directly with decision makers in a casual and non-threatening (salesy) way.
So, I'm curious. What say ye??