Certain programs that you get that they claim are "free" on their download page, but every time you open it up to use it, an annoying popup blocks your veiw of the program you're trying to use with something that says "BUY NOW!!" or "register Here" or something equally annoying. (even though it was claimed to be completely free).
one way to avoid nagware is to use a portable program.
nagware programs = WinRaR, LimeWire, mIRC, etc...
Software, released as freeware or shareware, that nags the user into purchasing, supporting, or upgrading on a regular basis.
Try downloading HTML-To-XYZ! its nagware, but it works great!