Shareware is a software distribution model in which a limited version of a commercial program is distributed for free as a demonstration for what the commercial release will be like. The term was especially popular in the early 1990s, but, since it was frequently synonymous with poor quality, the term fell out of use. Many computer video games in early-1990s used a shareware model where they would distribute the first couple levels of a game for free and include instructions for how to purchase the full game making it crippleware. However, occasionally developers would distribute the complete version of their program and just nag the user to pay money to eliminate the nag message, making it nagware.
I respect the shareware model because it is upfront about the fact that it is a limited version of a commercial program. I greatly prefer it over programs that call themselves free, but are actually crippleware, nagware, or adware in disguise.