There are many aspects to search engine optimization. From creating good content, to link building, to keyword optimiztion. This page includes some resources for people who want to increase the visibility of their website.
Link Building
Best of Free Internet Directories
DMOZ
DMOZ open directory project is one of the most well known and popular free directories around. Unfortunately, it's popularity also means that getting listed can be difficult. But if you are listed the link is very valuable.
My Green Corner
My Green Corner (PR 6) is has a number of listing types some of which are free. In addition to the directory it has a number of other resources for webmasters.
Most Popular Sites (MPS)
Most Popular Sites not only offers a directory but also rates the listed sites. You need to register in to submit your site. Once it has been accepted you can add a vote button to your site. The more people that vote for your site, the higher the rating on MPS.