(Rating: 5 out of 5)
The jQuery Library for SharePoint Web Services (SPServices) bundles into jQuery the vast methods and massive amounts of data contained within SharePoint. This space isn't big enough to mine all that wealth the last sentence contains. But, suffice it to say: everything that SharePoint knows about itself is exposed through its built-in web services. SPServices puts that knowledge at the fingertips of the user / site administrator / library maintainer.
Need the profile of the current user? There's a web service for that. Need list data from another site collection? There's a web service for that. Need permissions information, version data, alerts, etc.? There's a web service for that.
Need to WRITE data to a list, UPDATE permissions, MANAGE document workspaces and meetings, CREATE sites? There are web services for that. And they're ALL exposed through this library. Want to create a form that walks a user through creating a site based on your company's governance plan?
You could do that through SPServices. And never touch Visual Studio.