Uthman (ra) was a companion of Prophet Muhammad (PBUH) and later become the third caliph of Muslims. He is known because of many of his sacrifices for Islam which also includes donating and doing charity for the well being of the Muslim community.
In Medina there were limited sources of water, among them a well which was owned by a Jew who sold water at high prices. Uthman (Ra) purchased that well and declared it as a waqf for Muslims. This allowed Muslims to take water without paying anything for it. The well is still situated there and people still benefit from it.
When Prophet (PBUH) intended to extend the Masjid an-Nabawi Uthman (ra) was the one who purchased the adjacent lands from people to help the Prophet (PBUH).
During his caliphate he also renovated and beautified the Masjid with marbles and beautiful stones, it was the first time that the Prophet’s mosque was decorated.