From Campaign Monitor. Hugely helpful. It's interesting to see how bad Gmail is at handling emailers. Make sure to download the full reports to check out more.
Excellent, excellent resource. Mike Kleiman posts about various HTML email subjects, and has a 3-part series on how to code one. This links to the "Email" label, so you'll have to scroll to find the series.
From 1stWebDesigner.com. These are resources that every designer should have, cause you may not always have Photoshop on hand. I've been a fan of Pixlr for a while, but I'm definitely going to give the other editors a shot.
From SixRevisions. Especially notable for the inclusion of drop shadows and column counts. You can create multiple columns with older CSS (as you can see in my sidebar), but this is a great new way.