Utattemita (歌ってみた) is a Japanese term which literally means "tried to sing", and the English version of Nico Nico Douga translated it as "Sang_it". The utattemita category is where utaite upload covers of various songs. It is a subcategory of the Yattemita section on Nico Nico Douga. Most of the uploads under this category will have "song name を歌ってみた" title (or variations of such).
Yattemita (やってみた, which means "tried to do"), is a section in Nico Nico Douga where people upload videos of them "trying to do things". There are currently five subcategories in this section: utattemita (tried to sing), odottemita (tried to dance), egaitemita (tried to draw), ensoushitemita (tried to do a musical performance), and nico nico gijutsu-bu (arts and technology department).
Utattemita is different from Awasetemita (合わせてみた). The latter means a mashup of two or more renditions of a song. Therefore it is not an original upload of the singer. Rather, a person, often unrelated, though there are exceptions, took a few singers' covers and combined them to make a chorus-like version. Unlike with YouTube Choruses, the Utaite in them are generally not involved.