Bottleneck diagnostic tools

Hi,

Can anyone recommend a free/cheap diagnostic tool to check network throughput? The topology is as follows:

My site (site #1) connects to an offsite location (site #2) via a fiber ring. Both sites connect to the ring via a 50 mb connection.

Site #2 has a 50 mb microwave link that transmits data to a third location. When we recently downloaded files from site #1 to site #3 our transmission speed maxed out at 3 mbs. I need a utility to figure out where the bottleneck in this scenarion is occurring.

Any help would be greatly appreciated.