There is a lot of debate about whether submitting sites with descriptions to free online directories actually works for SEO. I've been doing it for quite a while for various sites. And I can assure you it definitely does work. It may not be the case in the future, but online directories still have some life left in them yet.
It's so simple to do. But it's also time consuming and boring. You can find companies who do mass submission. But this approach is a bit problematic, because it's hard to know exactly how Google treats mass submission. There is a widely held theory often expressed on various SEO forums that holds that you might get marked as a spammer if you use mass submission. It's probably unlikely that this would happen, unless you really went overboard with this approach. Still, this is definitely something to consider.
There are companies that hand submit to directories. Some may be dodgy, however, so it is advisable to buy a small package first and assess the results before purchasing one of the larger packages on offer.
Of course, you can do it yourself. If you just keep chipping away at it from time to time it doesn't become too exasperating and you're sure to stay in Google's good books.
If and when you do, here are some tips:
When you do submit your URLs and descriptions always make sure that you put them in the right category. Also, have a few different descriptions ready to go. And rewrite them a little every time you submit. (Google loves unique content, remember. And this applies even when you are writing short paragraphs.) Also, use different anchor text for each submission. You can target a whole range of keywords that way.
It takes a while but if you just keep at it you'll definitely see a gradual improvement in your search engine traffic.