If you just want the most traffic, then Digg and StumbleUpon are the route to go. If you want the best traffic (for any sort of conversions) then work the bookmarking sites like Delicious, Reddit and so on that will improve your SERPs and lead to targeted visitors.
Classified ads is another source. But again its more like a trial and error process to find whats best.
There are several smaller sites (Design Float) where the members are also members of other sites. If you website gets a high rating on Design Float people there will both bookmark it on Del.icio.us and Stumble it, if your lucky someone will even Digg it.