http://www.bandmusicpdf.org/ is my favorite site for band music. Most of their music is from the 1880s to the 1920s. They have thousands of pieces, many of which are for complete bands.

http://www.sheetmusicfox.com is a compilation of links to many other sites including Mutopia project and imslp.org.

http://www.classical-scores.com/free/ has over a thousand classical pieces for free. The majority of them are for piano, but their are also pieces for over twenty other instruments.

http://www.sheetmusicarchive.net/index.cfm has over 22,000 classical pieces. The majority of their pieces have a six dollars a month fee, unlimited downloads. However, many of the pieces are completely free. They also have a ten day unlimited download free trial time.

The above mentioned sites I have also found to have an excellent selection of sheet music. http://imslp.org/wiki/ however, is my absolute favorite.