DotNetNuke Tip: Validate your modules
One easy and useful thing you can do for your DotNetNuke modules is to validate them against W3C standards. I just spent the past 45 minutes (when I should have been sleeping) going through and validating the next release of our Engage: Publish module against http://validator.w3.org/
In most cases this is a very simple that you can do to help your customers by providing them validated code that will be browser and SEO friendly. The DotNetNuke Core team has set a mandate that all DNN modules submitted to be released after a certain date (I believe 8/08) have to validate against XHTML Transitional 1.0, I would recommend that all other DotNetNuke developers follow this same standard to try to raise the bar of the modules that are out there on the market!
You can also conduct this same validation against your website as a whole. The process I was using to validate the Publish module was to run the validation against my personal website which is powered by DNN and Engage: Publish. I was able to track down a lot of issues within the validation quickly and make those changes. There are still other issues with the site, but right now none of those are in the Skin itself or the Publish Module.