Polyatomic Cations

While learning about poly-atomic ions, our instructor stated that ammonium is one of the few poly-atomic cations. Are there other poly-atomic cations? And why are positively-charged poly-atomic ions "rarer" than negatively-charged poly-atomic ions?

Generating a simple polyatomic anion is easy: Take an element that is not fluorine, bond it to as many hydroxy groups as its valency is and deprotonate the hydrogens — voilà. You can then play around with oxidation states (i.e. add more oxygen) to generate even more anions, e.g. with $\ce{PO3^3-}$ and $\ce{PO4^3-}$.