Cool some nice features.
Here's a few ideas:
In the top right nav that extend when hovered add a js delay of about half a second before they close.
In the browse per category area just above the downloads you should make the whole div's clickable and linking to the section instead of just the small text link. Maybe add a hover for it then too.
Why not have the file hosts visible instead of having to hover as it's slightly slower having to hover on all the links.
Maybe you could work a bit of js magic so those hover images always appear on the screen. If you hover over a link at the top of the screen you don't see the images. A quick google will return a few scripts.
Add a roll over to the show download information to help people see it actually is a button. Maybe some nice CSS3 border radius to it too.
Why not let guests leave comments. Make them enter a capcha if necessary and moderate them.
You need to strip slashes after doing mysql_real_escape eg: "...but don\\\'t ever seem...."
In the comments why not have the "By: DeLeTeD on 2010-09-07" have the name link to his profile too. It will help google move around your site. I'm sure you already have the userid called so this should be really easy.
You could also move that "by name date" into the blue header of the comment and have a float:right; I think it would look and fit into the skin better their.
As it's a simple custom script you could do really cool features like let members login with their facebook accounts using their api script. I've being looking at this and it's not hard. Save people having to verify their email and you never really get spam from Facebook accounts as their really well policed.
Right that's enough for now. It's great work Whoo. I just think it needs a bit of polishing and know it's not fully finished eg the popup boxes are plain in areas. Keep it up