Music files - usually in mp3 format. This is often done illegally by websites that share files....but some websites, notably Napster have turned to legal distribution of music.

A user can pay for the music and then download it. Sometimes, the tracks of an album are individually priced, so users can make compilations of different artists.

Sometimes trial versions can be downloaded which expire after a certain time, or may not have all features enabled. After you pay for the software a code is given to you to unlock the software.

Shareware - Software distributed on the basis of an honour system. Most shareware is downloaded free of charge, but the author usually requests that you pay a small fee if you like the program and use it regularly

Some software allows free upgrades that can be downloaded over the Internet. Virus-checking programs normally download up-to-date lists of viruses on a regular basis - this is needed because new viruses are coming out all the time.