In Google Reader and MetaFilter (just two examples), the j and k keys navigate to the next item and previous item respectively. After falling to the addiction of using them on those sites, I want them on every site. Of course, not every site is structured the same, but blogs are very similar, and maybe paragraphs can end up being a point which the browser jumps to.
On first press: go to the top of the article, skipping over the rest of the site (headers, ads, etc.)
if it's a list of blog posts, subsequent j presses go to the next blog post
If looking at a single article, j/k navigates between paragraphs, then at the end, to the comments, and after that, between comments
This could happen at various levels:
at the browser level
at the extension/Greasemonkey level
ScrollMonkey looks promising, if it works. There's also a bookmarklet, but you have to click it each time you want it.
at the website level, via a plugin/module/etc.