Ad tools for free advertisements

I am looking for an add-on or resource that would best handle placement of ads within Certain node topics and forums.

I have some companies that would like free advertisements/sponsorship in exchange for content.

My forum is new and not yet really live so I would like to experiment with the design and placement of the ads before going live with them

What I am looking for is an add on for advertisement placement wherever I want it (whether in specific categories, nodes ,side bar etc).
I have the widgets add on. Cant figure out how to use it for the ads? If anyone has any suggestions, Please let me know. Still a Noob, but getting better.