Serial data plotting programs
There are some good applications for visualizing the serial data, including:


MegunoLink ($29.95; Lite free) - plotting, logging, programming, reporting and more. 


MakerPlot ($39) - digital &amp; analog plotting, monitoring, custom interfaces and more.


ArduinoPlot (free) - simple plotting.


arduino-plotter (free) - easy, light-weight plotting with support for all primitive types


Bridge Control Panel (free) - logging, plotting, and more.


RealtimePlotter (free) - 6 channels data plotting.


Real Time Serial Data Plotter (free) - CSV data plotting, logging and more.
SimPlot (free) - 4 channel plotting.


Arduino Serial Plotter (free) - built in Arduino Editor under Tools > Serial Plotter.




(This is a community wiki, you can expand the list.)