A straightforward but very useful idea, Corntab is a dynamic visual crontab generator for Unix and Linux servers produced by David Knell. If you've ever had to stop and check the manpage to get your crontab syntax correct (I find that I just don't create new entries often enough to keep the information in my head), you may find it quicker to use this Web site instead.
Just click through the various options for minute, hour, day, month, day of week and command, and your crontab entry will appear in the beige box at the top of the page. Cut and paste, and you're done.
To actually install the crontab, you'll need to run crontab -e (for your own crontab, or crontab -u juliet -e to specify a different user) then paste in the generated line. For a system-wide cronjob, edit /etc/crontab.
Remember, if your command has any output, you'll get it emailed to the user for whom the cronjob is running. If you don't want that to happen, add > /dev/null to the end of your command – although be warned that this may lose useful output. That syntax won't redirect error output, so you will still be emailed in the event of an actual error. It's usually a good idea to let the job run for the first few times without the redirection /dev/null. That way, you can get the emails and can check that all is going smoothly.
Juliet Kemp has been messing around with Linux systems, for financial reward and otherwise, for about a decade. She is also the author of "Linux System Administration Recipes: A Problem-Solution Approach" (Apress, 2009).