Getting a list of posts in a category in a specific order

How can I change the default order in which the posts that belong to a category are displayed?

For example I would like to get the most popular posts first?

Is there also a way I can retrieve all the posts and define the order afterwards? I'm using a plug-in to set the order of posts in a category and it doesn't allow me to change the order in which posts are retrieved.

Thanks.
Andrea