I just realized that there is no topic in here for people to post helpful links. Here are some sites that I've found that may be helpful to the visitors of this site:
http://w3c.org/ -
The place to find information on Transitional and Valid HTML, as well as just about everything else.
http://www.opensourcecms.com/ - A very nice site offering freeware, open-source PHP content management systems for many different applications. They have portals, blogs, e-commerce apps, and a lot more.
http://hotscripts.com/ - A decent site with a ton of applications in ASP, Perl, PHP, and a lot more.
http://sourceforge.net/ -
The place to find free, open-source software.
http://www.php.net/ - The online copy of the entire PHP documentation.
http://dev.mysql.com/doc/ - The official documentation for MySQL
https://www.pdfonline.com/convert_pdf.asp - A nice service to convert any document or image to PDF for free.