The various resource have varying degree of how they want to be cited in your game, some just want an email because they are curious how their assets are being used.
Open Game Art
Wrath Games: Free Development Resources
Pedro Alonso PablosPedro Alonso Pablos - Free Music (Sounds 8 bit)
Google Sketchup 3d Warehouse
You can use Google Warehouse with Sketchup to produce 2d images of 3d models.
This last one is a bit tricky to find the permission to use. This line comes from the Warehouse's Terms of Service:
"Rights granted to other end users of the Services. You give other end users of the Services a perpetual, sublicensable, irrevocable, worldwide, royalty-free, and non-exclusive license to reproduce, adapt, modify, translate, publish, publicly perform, publicly display and distribute Existing Geolocated Models, Existing Non-Geolocated Models, New Models and related content and derivative works thereof which you submit, post or display on or through, the Services."
Hope these are useful.