A dominant seventh chord as the fifth degree of a key (notated V7) is often used to build a cadence back to the tonic.
With that function in mind, I've seen people refer to other chords as "it functions as a V7", but without the chord itself being a V7. One example I know of is applying tritone substitution to the V7 chord, which changes the chord but not its function.
When composing, which other chords should I have in mind that function like a V7 but are no the V7 chord? If there are too many possibilities, which are the most common V7 substitutions in western music (pop, rock, jazz, classical)?