Are there any amphoteric substances other than water and ammonia?
First, it may be worth elucidating the nomenclature a bit and distinguishing between amphiprotic and amphoteric  substances. The former are those that can react as either acids or bases in the Brønsted-Lowry sense, while the latter are those that react in either manner per the broader Lewis definition. As you mention, water and ammonia are amphiprotic (and thus also amphoteric by definition), which is evident even from the fact that both undergo auto-ionization:

$$
\ce{H2O + H2O &lt;=&gt; H3O+ + OH-}\\
\ce{NH3 + NH3 &lt;=&gt; NH4+ + NH2-}
$$

In fact, a large number of protic substances exhibit this type of behavior to an appreciable degree, including concentrated solutions of various acids (e.g., sulfuric, hydrofluoric, etc.). Another canonical example of an extremely common amphiprotic molecule prevalent in nature is bicarbonate, which you find in countless consumer products, as well as within the human body as a physiologic buffering agent.

Under the right conditions, a very wide array of molecules of diverse types can be amphoteric. The two necessary and sufficient criteria for amphoterism are:


The molecule possesses lone pairs, so that it can act as a base.
The molecule can accept electron pairs, so that it can act an acid.


Those are not rare properties, hence amphoterism isn't really an exotic phenomenon.