Brassdragon- All things pennywhistle on the 'net. Home of the "Pennywhistle Matrix" and the "Pennywhistle Information eXchange". Tunes, reviews, news and overviews for pennywhistlers of all levels.
Ceolas Tin Whistle Guide- Overview of the tin whistle by Gerard Manning. Introduction to the instrument, resources, recordings, and purchasing information.
Chiff and Fipple- A general site by Dale Wisely, the "Undisputed King of Internet Tin Whistle Journalism". It contains a message board, a tune of the month, whistle reviews, and information and links for both novice and experienced whistlers.
Rob Greenway's Irish Flute Pages- Though the site is about flute technique, the description of vibrato, breathing, and ornamentation, is directly applicable to whistle as well.
The Tin Whistle Page- Music, instrument reviews, jokes, lists of books and tutorials, instructions for making your own whistle, and whistle artists.