There are lot of Drupal modules that makes the list almost impossible to browse and know. But, there are couple of modules that you can nt do without if you are there to make money out of your site
1. Adsense Moule and Adsense Injector: There are 2 separate google adsense mdoules that help you put your google ads in a most lovely manner in your Drupal content. But, sometimes I wish I could simpy use any ad and not just google like these google ads do.
2. Affliates: This is another module that can help you create affliates and never forget to ask people to promote you.
3. SimpleNews: Create a newsletter and keep people coming back to you.
4. Ad module: This one is useful when you want to display your own ads and you want to promote your other or partner sites. If you are ambitious kind, you want to this to act as a starting point to creating your own ad network for all your Drupal sites.
5. lm_paypal: Another key module with the help of which you can create paid subscriptions, donations, paid postings, paid classifieds using yourPaypal account.
And then there are whole lot of ecommerce modules that can help you sell your stuff online.