Occasionally you will encounter a situation in which you do not have a particular spirit or you run out of the necessary ingredients
Gin - Vodka
Gin can almost always be substituted for vodka, and vice versa. While these alcohols taste quite different, the texture and appearance of a cocktail is not altered by switching, and most often will be just as good.
Rye - Bourbon/Blended whiskey
This is a guideline only. Whiskeys have distinct flavors, even whiskeys of the same grain. Experiment, but keep notes about what works and what doesn't - especially if you have whiskey-loving guests.
White Tequila - Gold Tequila
Many "tropical" drinks such as Margaritas do not actually rely on the subtle distinctions between tequilas, as much as advertising would have you believe. However, be careful with this substitution in clear colored or strongly mixed cocktails.
Rum - Tequila
Exotic drinks tend to rely on spirits brewed in equatorial lands. Rum drinks often work well with tequila substitutions and vice versa, especially white rum <-> white tequila or gold/light rum <-> gold tequila. However, the flavour will not be the same, but it is likely the drink will still be good.
Schnapps - Liqueur
Sweeter brands of schnapps are practically liqueurs and may be substituted freely. Be careful of dry schnapps, which can be closer to a vodka.
Tonic Water - Carbonated Water (Club Soda)
These are NOT the same. Tonic water is flavored with quinine, which is the base for making bitters. Carbonated water (a.k.a. soda or seltzer) just has dissolved carbon gases for bubbles.
Scotch Whiskey - Any other Whiskey
There is generally no acceptable substitute for Scotch.