Art Song Central
Art Song Central focuses on sheet music for singers and voice teachers. The sheet music is for music in the public domain and is free to print. Emphasis is on classical and traditional repertoire for voice and piano. IPA transcriptions for songs in French, German, Italian, and Latin are also downloadable.
ChoralWiki
ChoralWiki has scores, texts, translations, and other useful materials for choral music in the public domain. Similar to IMSLP but with an obvious emphasis on choral and vocal ensemble music.
IMSLP Petrucci Music Library
The IMSLP (the International Music Score Library Project) is easily the largest free resource for sheet music. Everything from solo instrumental music to orchestral scores is available here (barring niche or copyrighted works). Scores can be browsed according to instrumentation, national origin, time period, or composers (among many other filters).