I’ve become a regular user of WhatsMyDNS.net. It makes it super easy to see how far your DNS changes have propagated after you make them:

Whats My DNS
Monitor Updates to Your DNS Changes

So, for example, if you are setting up a new site like FleeTheJungle.com and changing the A records to a new server, you can see when the DNS changes have propagated to your location.

I’ve found it very useful when working with new site configurations.