Re: What is your most in-demand service?
It sort of depends on what services you offer or what your area/s of specialty might be. I offer social media assistance so all of clients are within that realm. Within social media though, I would say the most in demand services would be Facebook, Twitter, blogging help and web research.
What I really like is that a good portion of my clients offer social media services themselves, mostly as higher end consultants, so I get the opportunity to help support them in the work they do AND learn more in-depth about using social media for business from them.