In addition to investigating security incidents, online research can reveal information about a person. If you were hiring someone for a sysadmin position at a major bank you would probably do some online searches to make sure they did not advocate that the rich be turned into Soylent Green, or that they felt stealing money from large companies was justifiable if you gave some to charity. The data is still on the hard drive in it's raw format, it just is not very easy to find. This is generally not too effective, but does show how easy it is. Much more advanced tools exist that use a variety of techniques to figure out what the data is, and relink it so you can access it easily.
Software does exist to wipe files and hard drives.
It will stop all but the most determined attackers, but if you are up against federal or international law enforcement there is still the possibility they can recover the data. The next step up in destroying data is to use hardware based methods. However, with modern hard drives you need a relatively strong magnet to wipe them effectively, so this method is more suited for tapes and floppy disks. You can also bend the platter, a good sledge hammer or a large pair of vicegrips is really all that is needed for this method. For media such as floppies, and tape drives, where the magnetic particles are held on a plastic film, immersion in strong acids, or burning them is usually quite effective.