As I've stated before, your web site's page rank is determined by the search engines and one of their top criteria is inbound links. However, if your link is on a site that automatically uses a nofollow code, search engines won't recognize or spider the link. Sites like Craigslist, MySpace and Facebook use nofollow coding to cut down on spam. Therefore, putting your link on a site like Craigslist will only produce some click through traffic, which is good, but not as good as increasing your page rank in the search engines.
There are a number of web sites who will allow you to add a link (or a list of links) for free. Many of these sites are called social bookmarking sites like Listable, Digg, Furl, Flickr, Propeller, Technorati, Slashdot, Searchles and Yahoo Bookmarks. If you're not sure whether or not a site is using the nofollow coding or not, right click on the page in question, choose View Source and do a search for "nofollow". If you see it before your links, then you know the search engines won't spider them.
Remember, when you get your site's link on a very popular web site, search engines conclude your site is popular, too! Use your time and resources wisely and add your links to as many spiderable web sites as possible.