One of the most important things when publishing a website is exposure, there are many ways of bringing your site to public knowledge, advertisement is one of them, but a more basic and necessary way of doing so is optimizing the website to search engines. Without SEO planning and implementation your site may stay in the hidden web for a long time. 
As developers we encounter a wide range of SEO tools, some are free, some are pricy and many are open source and provide certain flexibility, referenced at the end are some of my favorites.
Most of these tools offer some good advices on a live site, but at that point, these changes are too costly to implement.
 
The first thing a good SEO'd (Yep, new word :) site needs is a Validated HTML. Invalid HTML may cost you on your page rank, such issues can be prevented beforehand while coding the HTML, so making sure you follow the W3C standard is preferred.
For example, a lot of GUI developers are not aware of the significance of… placing one  H1 per page or placing H tags in an incremental order.
Trying to fix such issues will take time and could result a massive change to the CSS and design…  sometimes produce even more bugs, as if the ones you already have are not enough.
 
Since implementing these changes or fixes is complicated after publishing a site, its mostly left undone.
Let's put down some ground rules to avoid this expensive refactoring:
 
1.  All the developers should be aware of SEO basics. Project managers are not the only ones to be concerned with promoting the site.
2.   Consult a SEO professional at the early stages of the project, allow them to code-review the HTML and fix the problems early (Or do-it-yourself and learn some SEO!).
3.  Use HTML validation tools as much as possible.
4.  Create sitemaps and make sure every page can be viewed without AJAX.

Tools:
Xenu - http://home.snafu.de/tilman/xenulink.html - Xenu's Link Sleuth (TM) checks Web sites for broken links and resolve page's meta data.
http://validator.w3.org/ - Single page HTML validation service.
http://tools.seobook.com/firefox/seo-for-firefox.html - SEO toolbar for Firefox.
http://tools.seobook.com – SEO help, the DOs and DON'Ts of SEO and plenty of tools to help you with SEO.