Widgetbox also has a section where developers can submit their own widgets for others to download and use. They also have a feature that allows you to quickly manage widgets you’ve already installed.
Widgetbox is currently free and will probably always be. As the service matures, they’re likely to add more features that will be available only after paying a fee for that feature. It’s a really neat service, one that looks like it will definitely become more popular as word gets out.