Actors

The Internet Movie Database Search by Actor/Actress--most entries include a biography, filmography, (including television and film appearances) links, etc.
Yahoo's Actors and Actresses List Alphabetical list of links to "Official" websites (with actor or network affiliation) as well as many personal pages created by fans.

Commercials

Yahoo's Television: Commercials Links to sites about TV commercials, as well as companies that produce them.

TV Shows with Websites

Yahoo's Television: Shows Select a genre or category for a list of specific TV show websites. Or, you can click on "Search only in Shows" to do a search for a specific program. Note: TV Show links may come from a variety of sources -- they might be "Official" sites (issued by a network to promote a show, for example); links to entries in The Internet Movie Database or other sources; or personal webpages created by individuals to pay tribute to a particular TV Show.
Cinemedia's TV Shows List Select: Categories/TV/Shows for an extensive listing of websites devoted to specific tv shows. Like the Yahoo list, these links come from a variety of sources -- from networks to personal webpages.

Viewing Guides

TV Guide Online TV listings, articles, reviews, etc.
TV Now Search a month's worth of TV listings by actor, actress, or keyword.
Zap2It Guide to what's onscreen: TV, Movies, Internet