This is one of those simple things that can have a dramatic effect on your rankings.
You can download my report on Social Media Marketing for a detailed list of social bookmarking sites
Delicious has since added "no follow" tags which devalue the links you make back to your site, however this doesn't change the fact that the strategy works like a charm.
I've also found a tool that makes bookmarking on multiple sites a LOT easier. Have a look at BookMarking Daemon, it'll save you a ton of time and is well worth the low cost of the software.