The V3 Redirection service is one of the best services available. A redirection service lets you get another URL for your website. For instance, they would allow you to assign "http://www.thinkquest.org" to "http://come.to/thinkquest". That way, you would go to http://come.to/thinkquest and it would redirect you to http://www.thinkquest.org. It's very useful, especially for sites with big addresses.
Easily one of the most popular services, Guest World lets you set up a guestbook on your website. With a guestbook, people that have come to your site can sign the book and leave you messages. It's an excellent service.
Free CGI is another excellent service to use. They will automatically set up any CGI script that you want onto your website. Like the other services, they are definitely worth a try.
If you're looking for a service that wasn't listed here, try The Free Site for other services.