This is a collection of Loading Indicators, widely considered to be a good UX practice. Loading Indicators are a simple, but effective way of letting users know something is happening. A common pattern is an overlay with an animated indicator. This prevents users from clicking around and makes it obvious that the site is doing something of value.
Circular loaders can be placed in a variety of spots (modules, overlays, inline) making them extremely flexible. Here is an example from twitter:
These bars sit at the top of the view and as the page loads, a progress meter will fill the screen horizontally. You view an example on ESPN Soccer:
Loading Indicator Animations
Biskitty uses a rotating circle with an interior gradient.
The World Wide Leader in Sports rolled out a redesign and with it came a subtle, but elegant loader:
GameFly animates the circular piece of their logo:
Limeades for Learning
Great use of lime here!
New York Times
The Times uses a gothic "T" as their indicator.
PayPal uses a nice blue halo.
Great animation here, they repeatedly animate the lines from the logo to show activity.
YouTube is where the loading bar indicator started.