DR.TCP

DR.TCP is a neat utility that allows the Administrator to change the various properties in the TCP stack of the operating system. These properties consist of MTU size, TCP receive window, whether the system will send ACK's for received packets e.t.c. These settings can also be applied to individual network interface cards.

A good example where you might need such a program is when you need to change the MTU size when trying to send data over a VPN tunnel, which usually requires the maximum packet size to be less than 1,500 bytes.