There's a wide, wild world of whistledom out there, and I'm too lazy to re-create stuff that other people have already done well. 
Go exploring, take risks, and don't forget to write!

Useful websites, in no particular order:

• Praise Whistlers Abroad - My internet home community, as it were.  Includes a discussion board for people interested in whistle, flute and other instruments, used in personal or public worship.  A good set of links, too.

• Cyberhymnal - The authoritative source for hymns online (imho), with lyrics (all verses), history, midis, and music score in Noteworthy Composer format.

• The Whistle Shop - Whistles, flutes, fifes, and songbooks.  Also a very good beginner's section, including an online tutorial.  The Meg is my whistle of choice for groups of kids, and these folks have them for the frighteningly reasonable price of $4.00 each. Contact them by e-mail before you order quantities for educational purposes (links on their pages).

• Whistle and Drum - Fellow Coloradoan Blayne Chastain has a really nifty shop! Worth a look if you are whistle (or drum) shopping, or just browsing.

• Mack Hoover - Maker of very nice whistles, known for a quiet, sweet tone.  Mack, an all-around good guy, put voice to the idea of Praise Whistlers Abroad, and has supported and encouraged Whistle and Squeak.

• Chiff and Fipple - What is a whistle links page without a link to the venerable C&F? Dale Wisely's site is the 800-pound gorilla of pennywhistle websites.  A great resource, but you probably found C&F long ago. Dale actually published a letter of mine years ago in his newsletter.  Impressive, No?

• Chris Peterson's Traditional Music - This page is great! Tunes in midi for listening, and gif format for sheet music. No tablature, and some of the keys may be difficult to play on a D whistle. But, hey, if I can learn to read and transpose music...

• Google Tin Whistle Tune Directory - Very large directory of sites with pennywhistle tunes. Content is similar to the dmoz.org directory, plus a Google rating.

• ethnicwind.com - Nick Metcalf, a fellow PWA'er, makes some strikingly beautiful whistles. Now to save up...
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  Musician's Network - More information and gear than you can shake a whistle at! Here's the Instruments - Winds page.