per page /post sidebar html??

Hi I have searched all over the place and dont seem to be able to find a direct answer –

I want to be able to add custom HTML into the sidebar area on a per post and per page bases ( i want to link out to different pages/resources depending on the page/post being read)

Is this possible? I have reasonable HTML experience, limited CSS and I have had a play with PHP, but by no means proficient.

Any help or advice would be greatly appreciated

Thanks
Tony