http://codex.wordpress.org/Determining_Plugin_and_Content_Directories

Plugins
plugins_url()
plugin_dir_url()
plugin_dir_path()
plugin_basename()

Themes
get_template_directory_uri()
get_stylesheet_directory_uri()
get_stylesheet_uri()
get_theme_root_uri()
get_theme_root()
get_theme_roots()
get_stylesheet_directory()
get_template_directory()

Site Home
home_url()

WordPress

admin_url()
site_url()
content_url()
includes_url()
wp_upload_dir()

Multisite

get_admin_url()
get_home_url()
get_site_url()
network_admin_url()
network_site_url()
network_home_url()

Constants
WP_CONTENT_DIR // no trailing slash, full paths only
WP_CONTENT_URL // full url
WP_PLUGIN_DIR // full path, no trailing slash
WP_PLUGIN_URL // full url, no trailing slash

// Available per default in MS, not set in single site install
// Can be used in single site installs (as usual: at your own risk)
UPLOADS // uploads folder, relative to ABSPATH (for e.g.: /wp-content/uploads)

WordPress Directories:
home_url() Home URL http://www.example.com
site_url() Site directory URL http://www.example.com or http://www.example.com/wordpress
admin_url() Admin directory URL http://www.example.com/wp-admin
includes_url() Includes directory URL http://www.example.com/wp-includes
content_url() Content directory URL http://www.example.com/wp-content
plugins_url() Plugins directory URL http://www.example.com/wp-content/plugins
theme_url() Themes directory URL (#18302) http://www.example.com/wp-content/themes
wp_upload_dir() Upload directory URL (returns an array) http://www.example.com/wp-content/uploads