Xnetload displays a count and a graph of the traffic over a specified
network connection.

It either displays bytes/second or packets/second, based on the capabilities
of the kernel. On 2.1.x and later kernels (including the 2.2.x and 2.4.x
series) it displays bytes/s. It requires the /proc file system, or rather
/proc/net/dev which is only found in Linux.  So AFAIK this program is Linux
specific.