There are several advertising widgets for WordPress. Most are commercial as they want a piece of the action. There are a few free ones that are crippled commercial ones.
We use a 325 pixel wide sidebar on this site which fits the 300×250 medium banner ad. This size of banner is a very common format used by all banner ad agencies.
We use an Amazon native contextual ad but it has some problems analysing the page with some of our sites. We also use CPM ads which have their own issues.
The AdRotate plug-in seems to be adequate for most purposes. The free version can manage most agency ads fine and it can randomly show them, it does not seem to replace them as per settings suggesting they changed the feature without marking it as such.
Once some ads are added then groups need to be made. We name the groups by ad size which makes it less problematic to select the right size for the widget. The widget then can work with a specific ad or it can use a group which is how we are using the plug-in. By having a dedicated widget is slightly less tedious compared to using a basic text widget and pasting in the advertiser’s code.
MEKS AD WIDGET
Meks ad Widget can use the code from Amazon fine. It supports 125×125 and 300×250 sizes, but it has issues with custom sizes for footers.