Have you ever wondered what to do with that obscure long number that is sometimes seen posted with files that you download?  If you have ANY suspicion on whether a file is legitimate or not, you should use that number to make sure you are getting the file from the correct source.  Other than checking for trojan’d files and other dirty deeds you can also check to make sure you downloaded the WHOLE file.  Why waste a DVD by burning a corrupted ISO?  It’s easy enough to run the checksum so that you don’t have to guess if the file is good or not.  Open up a terminal and type:

md5 filename

See?  It’s dead simple.  Just make sure the number matches the one that is posted on the trusted site where you downloaded the file.  If you want to get really fancy with the process, there is an automator script available from apple called MD5 Checksum 1.0 that allows you to right click and check any file right in the finder.