Sight Singing is supposedly one of the most difficult challenges on the AP Music Theory test and stunts many students. However, this site promotes voice, ear, pitch, theory, and rhythm training. With enough practice, this site will prove to be very useful. :)
Sight Singing can be pretty difficult for people who aren't used to reading notes on the dot like a vocalist. At this site, you can either subscribe or practice for free (and improve such skills!).