I do (well used to do, I haven't worked in months) ABA Therapy with Children with Autism... I think that definitely impacts their life. I also used to work with adults on the spectrum and considering a lot of my co-workers were totally neglectful, I like to think that my work with them was life changing as I actually interacted with them as opposed to just spending my work day at the computer.
I also in the past have worked with emotionally disturbed adolescents as well as adolescents with behavorial issues.
As soon as I'm licensed to do counseling, I hope that will be life changing for my clients as well.
But I think even the smallest things can have a great impact on someone's life. For instance, every year I pick a name from those holiday giving trees at the mall for economically disadvantaged children. Ok, while buying a kid a toy or a piece of clothing isn't exactly sending them to college or taking away all their financial worries, I'd like to think that I've had a small part in at least making them happy for that day and that can be lifechanging.