Alright, I want to let people post on an HTML webpage I have made. I don't want it out to the side or anything, but I just want them to be able to directly post their comments and ideas and have those show up as part of the page after it's been posted.

How can I do this?

A blog allows you to do this.

I was trying to figure out the same thing. But the server that holds my site only deals with static pages. I found out that you need to have access to a dynamic server.
If you have a static site as well there is an alternative that I found out. There are loads of third party guest book extensions eg "http://htmlgear.lycos.com/specs/guest.html?60,10" is one. you need to put some html in with your and people can leave there messages. but the only prob with this is that they will put advertising on the guestbook.
I did'nt go for this option because of the ads.