When it comes to directories, there's a lot of conflicting information out there. Some people say they're worthless, while others say they're a must for any serious marketing campaign. I think that I'm in the middle here. If money is tight, I would pick one big paid directory (BOTW!) and then try a few free ones, but if money is no object...directories certainly won't hurt your visibility. Since fees are subject to change, refer to the sites themselves for the most current information.
Major Directories
As the Internet's oldest directory (and producer of the best shirts) BOTW is my top pick for the one directory I'd choose if I was limited financially. They offer a yearly charge as well as a one-time charge (which is the better option.) However, if your site doesn't have an English version available, you're out of luck. They also offer a Local option that can be bundled in with your regular directory submission, for an additional fee.
Yahoo offers a plan for non-adult content sites and for adult-content sites, charging much more for the adult sites. They also charge a recurring annual fee. Considering the reach that Yahoo has, this is an excellent directory if you aren't limited financially. The sites are categorized by subject and location, and they offer a very attractive system for managing your listing on your own.
DMOZ is an industry favorite and always has been, and it's free! Since it's free, don't expect the same level of service as you can with BOTW and Yahoo though, but hey, did I mention that it's free? Submissions can take as long as a few months to appear on sites that pull their data, so if you're interested in getting into a directory asap, this isn't the one for you. However, it's a respected listing, and you have nothing to lose by submitting your site.
Like BOTW and Yahoo, Business.com has an annual fee. They also have a stricter policy on sites they will accept, refusing to list gambling, adult, online pharmacy, retail tobacco, multi-level marketing sites, payday loans, plastic surgery, and others. Again, if money is no object and your site fits their criteria, you have nothing to lose. However, this would be my last pick for a big directory.
Specialized Directories
The three big search engines all have local listings, so I'd advise you to get your business listed on all three. They're free, and speaking personally, they can bring in a lot of relevant local traffic.
Local Listings
There are more, of course, so just search for "local directories" and see which ones look good.
Niche Directories
Depending upon your niche, you may get fantastic results with a specialized listing. As much as it may seem a cop-out not to list some, your best bet really is to search for a niche directory in your exact industry.
Free vs Paid
Submitting to paid directories can definitely add up, but you may get better results than if you went the totally free route. DMOZ is free but a listing can take ages to activate. You may pay Yahoo for a submission and get turned down. It's critical to read the service terms on any directory, BEFORE you submit. Ideally if money is right, I'd say pay for a BOTW listing, hope and pray for a DMOZ listing, and submit to a few free specialized directories.
Do it yourself or have someone else do it?
Since a directory submission really is not that difficult, I'd say do it yourself if you're comfortable with the web. The submission processes are rather simple and intuitive.
What Can You Expect From Directories?
Traffic, most importantly! Whether or not you tend to use directories to find a relevant site, know that many people do prefer this method. Also just getting your site listed anywhere is great for your branding purposes, of course. The more visibility you can get, the better you are situation to do well on the web.
To conclude, directories are a good part of any marketing strategy, but they aren't necessary in order to rank well. They're just another part of the package, but one that's definitely worth the investment of time and money.