There are thousands of directories on the internet. Some of which are obviously more popular than others, but that heavily depends on turn around time from the webmaster (as far as approvals go) and how many hits your site actually gains FROM the directories you’ve listed with. Joining random directories around the internet, while they will help as incoming links, there are many scripts out there who block your URL from being shown properly so that doesn’t help very much with your PR at all. I have my site linked in quite a few PR6 and even PR7 based directories, hence my reason behind maintaining a PR5 for as long as I have. It also helps to comment on other websites, join forums, and basically get involved anywhere you can throw in your URL that will bring in the most results.
My example from before, Alive Directory, is a fairly well established directory. It houses numerous links to everything from Arts to Science and proof that they’ve done very well for themselves is the fact that they’re a PR6 and in the top 20,000 websites on the Internet according to Alexa. Now, I would like to say that it’s easy to get yourself on this particular site, but like most good directories – this comes with a price tag. A basic listing on their website, with no real guarentee of any actual hits from it, will run you about $35 a year if you put a link back to them on your own website. Note this is for regular websites. If you happen to run a blog and are interested in joining their new blog directory, then this will run you about $30 with a link back.
You aren’t limited to paid directories though, I’m merely using this particular one as an example. There are thousands of sites out there that are completely free, consider looking into them. Even if you’re required to paste a link back on your own site – that’s fine – someone is offering a free service to link you, the least you can do is return the favor.