In the section for "Great Sites", you have a number of lines that are bracketed with
</li>. Groups of such list lines need to be themselves bracketed with
</ul> (for unordered lists) or
</ol> (ordered lists). For example:
Also, it looks like most of your image links to outside sites get their images from those sites. Depending on the load for those servers, you may see delays in downloading and displaying their images. To speed that up, copy the images to your own server and change your
<img src=... to reference your local copy.
In places where you do have
<ul> followed by stuff, then
</ul>, you often have other html tags, such as
<center> mixed in. This is a no-no.