DROPitTOme. To use it, you need to have a Dropbox account. Both are free. It has proved to be one of the best things for assignment submission and grading.
Before DROPitTOme, I simply set up an email account that students would use to send attachments. But now students can simply click on a DROPitTOme link I put on my class website, type in the password, and upload their assignment file. The files end up in a folder called DROPitTOme in my Dropbox, where I can easily grade them and move them to my external hard drive for storage.
Since all their files end up in the same place, I need a way to know who they're from. So I came up with this naming scheme: period_lastnamefirstinitial_assignment. So, if a student named Michael Bluth from second period is turning in an assignment called homefill, the file would be 2_BluthM_homefill. This way, when it comes time to grade, all my files are automatically organized by period and last name, the same way my grading is set up. It takes a bit of effort to get 7th graders to figure out the process, but worth it both for me and for them. No matter what industry they end up working in, they'll need to understand correct file format and correct file naming.