sntop is an ncurses-based top-esque console utility for monitoring the connectivity of network hosts, supporting various advanced features and released under the GPL.
sntop uses fping (ping is supported, too) to determine connectivity of hosts, specified in a config file, on a regular interval. the results are displayed in a top-like format.
advanced features include html generation of results for automatic web page, secure terminal mode, execution of external file on alarm (configurable for either host DOWN or any change in host status), color support, and both user and system-wide config files. sntop aims for support on all popular unices and platforms.
http://sourceforge.net/projects/sntop - the sourceforge projects page; where everything is.
ftp://sntop.sourceforge.net/pub/sntop/ - find releases old and new. to be safe, check sourceforge.