Quaver's Beyond Marvelous Curriculum for the entire school year (I gradually rolled it in last year, starting in October), but because I see my kids twice weekly, and because I obsessively customize any curriculum I use, AND because I spend a lot of time following the lead of my students, I'm constantly looking for new ideas anyway.
This is by no means a comprehensive list of online resources. These are simply the resources I've used the most, and a great jumping-off point for teachers looking for new ideas. Most blogs have a handy-dandy "blogroll" on the sidebar linking to THAT teacher's favorite blogs and resources for further exploration. So here are some of my favorites:
Pinterest! Pinterest is not just for knitters and artists. There are loads of music ed-related pinboards already created, or you can do a search. My "teaching ideas" board is linked as a starting point.
Music K-8 Idea Bank: Free database of user-submitted plans.
Music for Music Teachers: Nice collection of free sheet music aimed at educators and students.
My Music Class at NAfME: Free K-12 lesson plans for NAfME members.
ArtsEdge from the Kennedy Center: K-12 lesson plans in all the arts.
Smithsonian Folkways Tools for Teaching
Teachers Pay Teachers: Not a free resource, but generally EXTREMELY reasonably priced, and allows teachers to be paid directly for those materials they spent so many hours on.
Music Teacher blogs with loads of lesson plans:
Beth's Music Notes. One of the first I started using and an all-time favorite. Enormous repository of ideas and free resources for the elementary general music teacher.
Elementary Music Teacher Blog. That's pretty self-explanatory.
Emily's Kodaly Music. Not just for Kodaly teachers, lots of great ideas.
In My Music Class, a K-5 blog packed with information.
Tuesday Music, with loads of free lesson plans from an elementary-turned-K-12 educator. This links to her "how to use this blog" post.
Music with Mrs. Dennis–colorful and fun for K-5.
We <3 Music at HSES–great blogging of kids actually doing the lessons! K-5
Crescendo Music Education Blog–Free resources, blog of the store.
Rhythm and Glues–This teacher teaches both music and art!
Free song/sheet music databases:
The American Folk Song Collection–includes some lesson plans!
Dr. Uke–Every song has ukulele tabs in a nice, printable or projectable format.
8notes–Free sheet music in categories including recorder and ukulele.
IMSLP (Petrucci Music Library)–The mother of all public domain music resources. Best for classical music, orchestra.
I could go on all day, but I've got planning to do! I hope these are useful.