Well, if you're just looking for a ping, one of the best online (imho) would be at http://centralops.net . There are a few really great tools there to test your site/network, though, not speed test.
If you're looking from something that will give you loading times for different types of users, try http://www.netmechanic.com . Not only do they offer a speed test, they will show you how to speed up your site (using a tool called GifBot).
Hope those help!