Slashdot
The left column is sensitive to changes/bugs in the table layouting code.

Spiegel
Page layout is done with floats.

linux.com
nothing special about this one...

The acid test complicated CSS layout using floats

David Baron's DOM test suite
Tests that work perfectly so far: DocumentFragment, Document, Comment, Node, Text

more jscript tests

David Barons CSS test suite

Some pages with tests from mozilla.org:

- tests lots of pages
- collection of links to test pages