Using theme and plugin directories
There are official directories for themes and for plugins on WordPress.org. Although not every theme and plugin is available here, the ones that are here have been vetted by the community to some extent. Anything you download from these directories is likely to be relatively bug-free. Plugins and themes that you get from other sources can have malicious code, so be careful. You can also see what the community thinks of these downloads by looking at ratings, comments, and popularity.
Additionally, plugins in the Plugin Directory are automatically upgradable from within your WordPress Administration Panel, whereas other plugins have to be upgraded manually. We'll cover this in more detail in a later chapter. You can find the Theme Directory at http://wordpress.org/extend/themes/ (the following screenshot) and the Plugin Directory at http://wordpress.org/extend/plugins/.