Here are some resources to address the following two parts of your question and allow you to identify some sites you'd consider good/bad:

1) "I want to show some colleagues the difference between good web writing and just writing"

• You could pick a variety of sites and use the 5-Second Tests for Measuring a Site's Content Pages to have them evaluate which are successful and which are not (getting them to find the good and bad could be even more effective than you pointing it out to them)

2) Re: "My emphasis is to explain that writing on the web is different than writing for print", you could refer to the following:

• Writing for the web (Nielsen)
• 10 Things to Consider when Writing for the Web (Carsonified)
• 23 writing and content quality usability guidelines
• As an add-on to Boon Chew's response re: Ginny Redish's Writing Web Content that Works, here is a blog post by Jeff Parks about Ginny's book, that includes a link to download 2 chapters of the book

I like texts here: http://www.flickr.com/tour/ (and in fact anywhere at flickr)

Maybe this one: Copyblogger

Woot.com always has something interesting to say about any given product.

Lee,

I think "well-written content" really depends on what kind of site you're talking about. Every site has a different audience, and as such, the content should reflect that (e-commerce sites can vary greatly too).

I would suggest doing some research around Content Strategy, by looking at books like:

• Letting Go of the Words, by Janice Redish
• Content Strategy for the Web, by Kristina Halvorson
• Don't Make Me Think, by Steve Krug

I believe even Jakob Nielsen has some useful nuggets about writing content for the web. He publishes some useful articles at useit.org.

Here's a question posted by someone else related to content strategy that might help: http://uxexchange.com/questions/1508/content-strategy-resources

The writing on Zappos is perfect for an e-commerce site - there isn't any. Well, hardly any.

Threadless and Vimeo are good examples for unique tone and good copy, in my opinion.