These species usually do not exist in nature, but they can be synthesized.
- Silver has been reduced in liquid ammonia to give $\ce{Ag-}$.
- A lot of anionic metal carbonyl complexes $\ce{M(CO)_{n}^{m-}}$ have been synthesized:
###-1
- $\ce{[V(CO)6]-}$, $\ce{[Nb(CO)6]-}$, $\ce{[Ta(CO)6]-}$, $\ce{[Mn(CO)5]-}$, $\ce{[Ir(CO)4]-}$, $\ce{[Co(CO)4]-}$, $\ce{[Rh(CO)4]-}$
-2
- $\ce{[Ti(CO6)]^{2-}}$, $\ce{[Zr(CO6)]^{2-}}$, $\ce{[Hf(CO6)]^{2-}}$, $\ce{[Fe(CO4)]^{2-}}$, $\ce{[Ru(CO4)]^{2-}}$, $\ce{[Os(CO4)]^{2-}}$
###-3
- $\ce{[V(CO5)]^{3-}}$, $\ce{[Nb(CO5)]^{3-}}$, $\ce{[Ta(CO5)]^{3-}}$, $\ce{[Mn(CO4)]^{3-}}$, $\ce{[Re(CO4)]^{3-}}$
###-4
- $\ce{[Cr(CO4)]^{4-}}$, $\ce{[Mo(CO4)]^{4-}}$
There's a review by John E. Ellis: Highly Reduced Metal Carbonyl Anions: Synthesis, Characterization, and Chemical Properties on these compounds.