Want to remove a file from your Mac, and have some confidence that it'll be very hard or impossible to recover that file later on? OSX includes the srm utility, which is quite useful for this purpose.
srm somefile.txt
The default is to overwrite your file with 35 passes of random data, as prescribed by the Gutmann method. This can take quite a while, so the slightly less paranoid among you can use the 7-pass --medium option.
srm --medium somefile.txt
Keep in mind that this isn't a fast process. I sold an external hard drive a while ago, and used srm to wipe the contents. It took about 2 days to completely wipe the 120GB of contents using --medium.
This StackOverflow question on secure disk erasure methods has a few other interesting comments and references.
