Mac: While Macs have a built-in, secure file-deletion system, for some people that might not be enough. If you're looking for a truly secure way to get rid of files from your computer, FileShredder is an app that offers just that.
FileShredder is a pretty no-frills app, but it offers several ways to securely delete files. A one-pass normal deletion that just overwrites your files, a seven-pass DoD compliant pass makes sure most people can't get into your files, and a 35-pass Gutmann deletion ensures that your files are truly gone when you delete them.
Each of the various passes have their own set of benefits. For most people this might be a bit overkill, but if you're looking for a cheap way to securely delete files FileShredder gets the job done.