If you use Mac OS X and you always want to be able to see your Mac's network and hard disk activity, I highly recommend DiskSpy Solid. The US$5.00 shareware puts 2 indicators — 1 for network activity and 1 for disk activity — in your menubar. The indicator on the left is network activity and the indicator on the right is disk activity (in snapshot above, network is 3rd icon from left and disk is 4th from left). The indicator tells you whether network data is flowing in or out (or when data is being written to or read from disk).

You can customize the look of the indicators (or use your own) simply by using different TIFF files in the program's resource. Instructions and 9 sets of indicators (the author calls them "themes") are provided. I am using the Arrows theme but I swapped the DiskSpyRead.tiff and the DiskSpyWrite.tiff files so that when data from the network is flowing in, the arrow points down (default is to point up) and vice versa. DiskSpy (no "Solid") is freeware but only shows disk activity.