i found another free useful tool today: nobodycheck.sh.
the file can be accessed right over here.
this tool will search for malicious files on your server and email you in the event that such files are found. right now, it's intended to be used by cpanel users, although the application for plesk and other web administrator consoles are in development from what i have heard.
there are never too many security tools that you can install on your linux server, as each can do something different and may catch something that another does not.