I scourged the web today looking for the very best (and free) tips on increasing a site's PageRank, and I figure it basically comes down to these simple tips:

Top 5

1. Get lots of backlinks from quality sites.
   One way to do this is to write articles and submit them to websites accepting author submissions. You could just google for these sites. Also it's good to inlcude your keywords in the anchor text from the sites linking to you.
2. Add your url to the Open Directory Project
3. Sign up for Google's Webmaster Tools.
   So you can learn more about how your site is crawled and to submit your sitemap.xml
4. Get W3C validated so a bot cannot be confused by bad code.
5. Use clear URLs for your pages.
   /title-of-post is much more descriptive than /?post_id=23452 for the crawlers as well as the visitors. I've also written an how-to on this subject, called Beautify URLs.

Some additional, less mindblowing tips

1. Start a blog!
2. Make sure you keep your content fresh and intereresting.
   Content is the best advertisement because if visitors love your content, then they will come back and spread the word as well.
3. Use descriptive title tags.
   Focus should be placed on simple, well-described page titles.
4. Put your keywords in your meta tags, header, opening paragraph, and footer
5. Make sure you're not inadvertently telling the search engine spiders not to index your files, via robots.txt, metas or htaccess files
6. Make it easy to recommend your site.
   There are tons of scripts freely available if you do not want to write them yourself.

I hope this list will save you some time. Did I miss any?