In absolutely no particular order, we begin..
Note that there isn't too much information about these people, other than what they sound like. They're also known as うたってみた, utatte mita (tried to sing). This is just a short list of a few, and there are many, many more singers out on NND. DO NOT THINK THAT THIS IS EVEN A GOOD CHUNK OF THE SINGERS OUT THERE. Some of them put professional singers to shame ^^;

anima/あにま (初めての恋が終わる時 ver.あにま)


anima's voice is similar to Ren's, but he's done covers of songs by most of the Vocaloid characters.

clear (初めての恋が終わる時 ver.Clear)


Clear's voice is similar to the ranges of Kaito and Ren, thus he is often portrayed as Kaito (sometimes Ren). He is one of the more well known and popular singers.

Halyosy


The lead vocalist of the professional band, Absorb. He often does covers of Vocaloid songs on NND. He's one of the few male vocalists able to hit extremely high notes. As a professional singer, he puts a lot of emotion into his singing. This is a perfect example. Wells up the tears nearer to the end.

nayuta (letter song ver. nayuta)


Her voice is in the range of Miku and Rin. While it is high pitched, it has a sweet, relaxing tone to it. This song is a perfect example.

Chouchou/ちょうちょ (Last Night, Good Night ver ちょうちょ)


Her voice range is similar to Meiko and Luka's. She is one of the more well known and popular female singers. She has a good voice, and her singing ability is possibly one of the best among the singers.

Usa (Last Night, Good Night ver. Usa)


Her voice range is similar to Miku's and Luka's. Her English is one of the better ones among the singers. Like ちょうちょ, she has a good voice and is an excellent singer.

Jack/じゃっく (千年の独奏歌)


His voice range in similar to Kaito's, and is usually portrayed as Kaito. He has as radio show (you'll have to find it yourself because I can't), where he will sometimes do covers of songs. He has several duets with Chouchou

Piko/ピコ (Hope and Despair)


A popular male singer on NND. He's usually portrayed by a doll. He has a strong voice, and is an excellent singer. He also has done an excellent cover of Don't Say Lazy!. I'd say his popularity rivals clear's (though I prefer Piko over clear ;D)

Dasuko/蛇足 (いろは唄ver.蛇足)


His voice range is similar to Kaito and Gakupo's. His is almost always portrayed as Gakupo, due to the similarity in voice. I personally think that he sounds a lot like Gackt (and Gakupo in that case). He's fairly popular, though he may be most famous for Imitation Black, a collaboration with Valshe and clear.

Valshe/バルシェ (Soundless Voice)


Her voice is similar to Luka, though she can also do a very good Ren. While she does covers of other songs, her Ren covers are usually the more popular.

Nodoame/のど飴


I honestly haven't heard much about him around. This was the first song I've heard from him. From what I've heard though, he's got a powerful voice, and is best suited for high energy rock songs. He seems fairly popular.