I recently found out a very useful SEO stuff supported by Google (here), Microsoft Live Search (here), Yahoo (here) and Ask.com ( here) – canonical links.
Basically, if you have a page accessible by multiple URLs, from search engine’s prospective you’re bloating its database by serving duplicate content. So to avoid this, you have to set <link rel="canonical" href="..."> with href pointing to the original URL of this page. Search engines will check the url in href attr and won’t put a duplicate content penalty on your pages.
Useful thing to remember and use!
WordPress (here) and RoR ( here) have already got plugins for canonical URL support. Hope to see support for this useful rel in other frameworks, forum and blog engines.
And I just found an interesting post on this topic by Anne van Kesteren with interesting discussion going in comments.