Oh my, this is my thread.
Alright, there are a TON of producers that do all sorts of genres, and you might prefer some Vocaloids (Vocaloid Libraries) to others.
Personally, I listen to mainly Miku, Rin, Luka (esp. with metal (p. yuyoyuppe)), IA, and Lapis.
There are also iterations (VOCALOID 1,2,3,4) which are basically just more advanced versions (re-recorded artists, tweaking, etc.)
Although, producers may take the liberties to fine tune to their liking using other software. Mitchie M is notable for making Miku sound very "human" w/ gradual note changes and vibrato.
There are also English, Korean, Tagalog, etc. ones, but you're not going to find nearly as many tracks for those languages compared to Japanese.
A good place to start is probably with the EXIT TUNES album compilations that feature a ton of different producers per album.
http://vocaloid.wikia.com/wiki/EXIT_TUNES_album_compilations
From there you might be able to find a producer that you like and branch out. (Check out covers of existing songs as well!)
Other than that, you might have to do a bit of YouTube or NicoNicoDouga (the holy grail/origin of Vocaloid stuff) digging if you want more than electronic/pop and soft rock.
There are also rhythm games like Project Diva PSP/Vita (which I play religiously), Cytus (mobile), Project Mirai for 3DS, etc.
I'll go ahead and put some of my favorites on here as well that you might be interested in.
I haven't really delved into the new V4 libraries, but I should probably do so pretty soon =P
[Pop] FREELY TOMORROW - Mitchie M (Miku)
[Pop] Promise - samfree (Miku + Rin) (R.I.P )
[Metal] Story of Hope - ゆよゆっぺ (yuyoyuppe) (Luka)
[Pop] Tell Your World - kz (livetune) (Miku)
[Pop] meltdown - kuma (lyric) | iroha (comp.) (Rin)
[Electronica] DYE - AVTechNO (Luka) | This song gives me the chills... Awesome Project Diva track (PD Extend | PSP).
[Pop] Melancholic - Junky (Rin) | Shougo from Anfiel covered this song, and I went comp. bonkers. Another favorite PD track.
[Pop] Daydream Flight - i-style Project (Lapis)
[Pop/Electronica] Far Away - kz (livetune) (Miku)
[Pop (Slow)] Star Story - kz (livetune)
Whew, there's like a million more songs. I'm personally not into the male Vocaloids unfortunately (other than a few Rin/Len duets.)
I'll probably expand on this since I don't feel like I covered enough XD. Also, other people can obviously chip in as well.
Enjoy!
Edit: Also Deadball P. is one of my favorites, I would include vids but I'll be here forever typing o_o.
Edit 2: Utsu-P has a lot of Metal/Screamo stuff that's pretty neat. I'm not too crazy about Vocaloid "harsh vocals" though (sounds like weezing, lol.)