trenchant.org - daily 
Fun With W3's Validator
Fun with the World Wide Web Consortioum's HTML Validator The Web Standards Project
"Congratulations, this document validates as HTML 4.01 Transitional!"
Jeffrey Zeldman's Daily Report
"Sorry, this document does not validate as HTML 4.01 Transitional." - 13 errors
Tim Bray's Textuality
"Sorry, I can't validate this document."
Steve Champeon's A Jaundiced Eye
"Sorry, this document does not validate as XHTML 1.0 Transitional." - 1 error
The homepage for HTML & XHTML: The Definitive Guide 4th Edition, published by O'Reilly
"Sorry, this document does not validate as HTML 4.01 Transitional." - 7 errors
O'Reilly Network - "The Source for Open and Emerging Technologies"
"Sorry, this document does not validate as HTML 4.0 Transitional." - 150 Errors
Lance Arthur's Glassdog Wed Design
"Sorry, this document does not validate as HTML 4.0 Transitional." - 27 Errors
"Sorry, this document does not validate as HTML 4.01 Transitional." - 112 errors
Jakob Nielsen's useit.com
"Sorry, this document does not validate as HTML 4.0 Transitional." - 11 errors
Cameron Barrett's Camworld
"Sorry, this document does not validate as HTML 3.2." - 71 errors
Peter Merholz's Peterme
"Sorry, this document does not validate as HTML 3.2." - 8 errors
"Sorry, this document does not validate as HTML 3.2." - 153 errors
Dave Winer's Scripting News
"Sorry, this document does not validate as HTML 4.01 Transitional" - 219 errors
"Congratulations, this document validates as HTML 4.0 Transitional!"
Notes:
- Valdations were done throughout this week, so actual numbers may be slightly different now.
- No, I didn't actually count the errors by hand, I just saved the results page locally and changed the unordered list of errors to an ordered one. Because computers count much better than I do.
