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 & analog plotting, monitoring, custom interfaces and more.
ArduinoPlot (free) - simple plotting.
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.
(This is a community wiki, you can expand the list.)