What am I, chopped liver?

Expression used to lightly point out to someone else that they are forgetting or underestimating you. Apparently chopped liver is an insignificant thing to be.

No - the reason chopped liver is used in the analogy (correct usage, "So? Vat am I? Chopped livah?," accompanied by half shrug) is that it is omnipresent*, therefore easily taken for granted. A subtle, but important difference. In correct usage the saying implies the sayer's sense of self-significance and conveys an appropriate load of guilt for one so thoughtless as to forget. The sayee is to infer that he/she is real schmuck for taking one so significant for granted.

^{*Or was, in the old days, before The Most Expensively-Catered Bar Mitzvah Spread Wins (shhh - don't mention we got wholesale!) tradition got started.}