@Zereo: There are minimal effects of the Ping on the down/upload speeds.
As example, extimating your network speed with a 100 byte packet will be VERY affected by the ping, as the packet will be sent at once.
If the ping's 19, result will be decent.
If the ping's 200, result will be sucky.
No one would do such a test anyways, that's very unreliable.
@Computerquip: Sucky. I'm able to access that website right away.
@Daleth: As you said, it's the RTT: Round Trip Time.
From what I know, servers like IRC do it on their own like follows:
Generate the current second on your PC (time(0))
Receive it back
Compare it with a re-generated current second.
Send it back
This is a really easy way to do it, and it can be as precise as you wish (You may want to use performance timers too on Windows? You can), but being too precise, results will be affected from external factors (CPU/RAM usage and so on).