How would I go about putting ads in my sidebar and posts (non adsense)

I’ve noticed that alot of sites are putting ads (non-adsense) in their sidebars and in their posts. On example is readwriteweb.com … You can see that the ads are everywhere almost…

My question : how would they go about managing all of this ? Are they putting it in by code ? Or is there somekind of ads manager for this ? Or is it easier to just put the code into a text widget. ?

Grs,
-FOP-