What salts (or other substances) are available for increasing water's boiling point?
For relatively dilute solutions of non-volatile solutes, the following equation (which assumes ideal behavior) can be used to predict boiling point elevation with reasonable accuracy:

$$\Delta T_b = K_b \cdot b \cdot i$$

Here, $K_b$ is the solvent-specific ebullioscopic constant, $b$ is the molality of the solute, and $i$ is the so-called van 't Hoff factor, which accounts for the extent of solute dissociation. For example, if one were to dissolve $\ce{NaCl}$ in water and assume total dissociation, $i = 2$, since two ions are produced in solution. It follows that, ceteris paribus, a salt that dissociates to yield a greater number of ions will produce a greater boiling point elevation. In practice, the magnitude of boiling point elevation is capped by solubility and deviation from ideal behavior at concentration extremes.

Water also forms negative (i.e., higher-boiling) azeotropes with various substances. Addition of a miscible higher-boiling liquid compound can also effect elevation of the boiling point (assuming the mixture does not form a positive azeotrope), a colligative property similar to the above case for salts. Various polyols (e.g., glycerol, propylene glycol, etc.) are commonly used for such applications.