Dreamwhittler's basic description is an HTML and CSS optimizer, but it also has many useful features which make it a one stop complimentary application to any HTML editor.
HTML
Dreamwhittler uses many techniques that will polish up your HTML, and can substantially reduce file sizes depending on how optimized the HTML is already.
In addition to faster page load times, you have the option of removing all tabs, white space, and carriage returns from your HTML, leaving your server side code virtually unreadable to the casual observer without a considerable amount of effort.
CSS
Dreamwhittler uses many separate techniques in addition to those listed above for HTML to optimize your style sheets as well.
Depending on how often you have used classes or ids in your HTML, this can also significantly reduce code bloat.
*Dreamweaver
When working with Dreamweaver templates, nested templates, and repeating regions Dreamweaver inserts comment lines into the html in order to identify said regions. These entries are invisible and useless to the browser that interprets the HTML, but they are sent to it anyways if included in the HTML uploaded to your server.
These comments do not usually take up an incredible amount of space. For example, the comments on this simple page are approximately ~1KB. But little things add up. If 1024 people look at only this page and none other on the site, that is 1MB in bandwidth that is paid for, and slowing down the time it takes to load the page. If they were to navigate to any other pages, the amount goes up very quickly.
That's where Dreamwhittler comes in. After saving your site locally, it will copy all files into a temporary directory, remove all the comments and upload the new files to your FTP server.
*Dreamweaver is a TM of Macromedia.