This additional instrument may be a viola, cello or double bass.
The other two combinations have far fewer important works, though the combination with two cellos includes the string quintet by Schubert, which is widely considered the greatest of all string quintets.
These lists should be complete for major composers and their string quintets.
For secondary or minor composers, the lists are not exhaustive but those shown may be taken as representative examples.
For recent or living composers, no attempt has been made at completeness.