My situation is as follows: I am approaching the completion of my PhD and actively hunting for postdoc opportunities. While I am confident in my publication record and general "credentials", I know that with this sort of things reference letters can be quite important. My advisor will write act as a reference writer for me and, given his evaluations of my work, I believe he will be quite positive...but I can't think of a single other person who will write a letter that could accurately portray who I am/how I work/etc.
The situation I am in is somewhat unique, at least given what I have gleaned from my friends and colleagues. I believe largely stems from the fact that the group I am in is quite young, the research we do is very different from that done by others at our department and university, and my lab has no collaborators outside of my university. This is to say that outside of the postdocs and other graduate students in my lab, I haven't directly worked with anyone else.
I could ask my other committee members. They know my project well and are seemingly nice enough people to be willing to do it, but they haven't actually worked with me or supervised any of my work. As mentioned earlier, the research they do is quite different from the research that my current group does and that of the labs in which I am applying. But perhaps I am just talking myself out of a good option that is right in front of me.
I understand that this is perhaps something I should have anticipated and, had I done so, I could have taken steps to initiate collaborations independently from my advisor during my PhD, but alas I did not and here I am.