8notes.com – Sort by artist, title, difficulty, popularity, style.
Yahoo! Voices – Easy Piano Pieces (“Easy” is harder than “Beginner”)
GMajorMusicTheory.org – Beginner Music (Click the green dots in the Adobe Acrobat column for free sheet music.)
MusicNotes.com – They are mostly a “pay for music” site, but they always have free stuff available.
Kate’s Sheet Music Transcriptions – Songs I’ve transcribed for my students. Mostly for beginners, and they are ALL popular or traditional songs, so you can look for something you know.