This section is mainly about getting your website listed at places where any URL is accepted. The only check might be that the URL really exists. In some cases a certain selection takes place, especially by Web Directories and Link Exchanges.
Submission to the places discussed here is mostly just a matter of sending the URL. In some cases a title and description of the website and some personal data (like email) are requested.
Much has been written about exposing your website, and many free services are being offered. I don't have much to add to this. So this section is mainly a lot of links.
search engines and web directories
In Web Site Marketing for Realtors it is stated that 95% of the visitors find you by search engines. That might count for commercial sites. I think that links to your site are more important, but I can not really prove this (and I have no intention to prove it). If you want to know what people are looking for using search engines, check the Yahoo list of most used keywords.
Before submitting your website to a search engine, read the FAQ of the search engine, it offers you important tips. Anyway, include metatags in your page. These are used by some search engines to index your site. Especially if you use frames this is important.
Some general documents about how search engines work are available. It is useful to read some of this stuff when promoting your website.
Here are some lists of search engines
There are some special search engines and utilities. For example showing at which position search engines show a URL on a search on keywords. And a "ranking" search engine, presenting the most popular websites on a keyword search.
free lists and what's-new sites
There are many lists available of "add your URL-" and "What's New-" sites. Check them out.
There are also temporary lists. These put your URL at the top when you add it. As others add their URL, yours will slowly move to the bottom until it disappears from the list.
Link exchanges make that a link to your site is being shown on on websites of other members. In return you have to advertize someone else's website.
These link exchanges usually make use of banners, so you'll have to make a banner for your site. Even for that support sites are available.
A mailing lists is a list a email addresses to which regurlarly a message is send. It is very interesting to get your URL mentioned in a mailing list: all subscribers of the list will get the message.
Just like in newsgroups (see below) there are 2 kinds of mailing lists: moderated and non-moderated. On a moderated mailing lists there is just one person (or organization) sending out the mailings to all subscribers. When you want your website mentioned in such a mailing list, you will have to convince the sender of the mailings to mention it. On other mailing lists anyone (or maybe any subscriber) can send a mail to an email address and then it is send automatically to all subscribers. Of course there are some protection mechanisms against persons sending unwanted mail. And it is the question if you really promote your website by sending out mails to people who are just annoyed by your mail. It only works when your website is really interesting for the audience of of the mailing list.
Newsgroups (Usenet) are a very effective way to promote your website. There are numerous newsgroups on each subject you can think of. Many newsgroups are moderated, which means that any message posted is judged by the moderator before it is placed in the newsgroup.
Even if a newsgroups is not moderated, the regular readers will not appreciate just any message and might "flame" you when you post a message which is not about the subject of the newsgroup. So before posting: read the FAQ of the newsgroup.
Another effect of posting a message in a newsgroup might be that you will get a lot of junkmail afterwards. There are programs running to collect email addresses from newsgroups. The good thing is though that you can just throw these messages away.
There is one (moderated) newsgroup especially meant to announce websites. Read the FAQ before posting there.
Virtual worlds are mostly just what is says: big and easy to get lost in. They offer you a place for your home. The purpose of virtual worlds is to meet people, and visit their homes. You can put a link to your website in your own home.
If you do not very carefully register the coordinates of your own home, you might have problems to find your own home back. The chance that others find your home and the link to your website seems quite small. But who knows..
Guides, services and tools
There are places that help you submitting your URL to many search engines, web directories, free link lists etc. Some sites are like this one, with tips, hints and links. Other offer a service for submission, after you have given some essential information to them, they will send this information out to search engines, free link lists etc.