There are different useful resources for wordpress plugins.

The following urls are links to wordpress plugins:

http://wiki.wordpress.org/Plugin,
http://dev.wp-plugins.org/wiki/PluginDirectory

A really cool plugin is the WP Plugin Manager. It provides a one-click installation feature for some plugins and contains an update function, where it automatically checks, if there are newer plugin versions available.

For german speaking users the german portal http://wordpress.de/ is also worth a visit.