The following sites contain free interfaces available for download:

http://www.freewebtemplates.com - Besides web templates this site also contains some really professional, and really awesome interfaces. Definately worth taking a look at.

http://www.andyart.com - This website provides tips to making interfaces. Somettimes this best thing to do is create an interface from scratch.

http://www.designsbymark.com - Also contains some really helpful interface tips. I learned many new things from his tip section. He has flash, illustrator, and photoshop tips. I strongly suggest you look into this site if your serious about learning photoshop.