The Last Alliance of Elves and Men was that alliance between Gil-Galad of the Elves and Elendil of Númenor between the years 3430 and 3441 at the end of the Second Age.

The Last Alliance marched on Mordor, besieged Barad-dûr, and defeated Sauron. During the final battle of the Alliance on the slopes of Orodruin, Gil-Galad and Elendil, the father of Isildur, were killed. However, by his death, Gil-Galad enabled Elendil to strike down Sauron before his own death. Isildur cut the One Ring from Sauron's hand with Narsil, but would not destroy it. In this way, the ancient essence of Sauron's power survived his bodily defeat, and the sacrifices of the Last Alliance were rendered in vain.

The actual army that marched on Mordor seems to have consisted of more than Elves and Men. There is at least a record of some of the Dwarves of Durin's Folk fighting with the Alliance.
Encyclopedia entry originally written by Sister_of_Legolas