When a web application needs to perform a long calculation you don't want to wait for a result. Celery is a task queue which can perform the calculation in the background or asynchronously. Celery can handle many calculations as they will be queued and distributed to set of workers.
Celery is used in the eEcology project. The 'Tracker Calendar', 'KMZ Generator' and 'Flight Generator' virtual labs on http://www.uva-bits.nl/virtual-lab/ all use Celery to submit job request, follow progress and show results.