Advertising has to be targeted to work.
So here are a few services that might work for indie games.

Reddit

Comic That Explains Reddit Advertising

• Pay per day model
• Bidding model (20$ per day for single subreddit or 30$ for site wide)
• Ad will be displayed on targeted subreddits or site wide (it’s your choice)

I think reddit advertising is pretty effective because of the very targeted subreddits.
Even I click sometimes on ads there…
For more information consider to check out this subreddit called self-serve sponsored links on reddit.
(People share their experiences with reddit ads)

By the way here is my reddit profile and i mostly visit IndieGaming and GameDev.
Moreover here is the Big List Of Gaming Subreddits.

Project Wonderful

• Pay per day model
• Bidding Model. Some offer ad space for 0.10$ per day some offer it for 4$.
• Ad will be displayed on choosen sites from the Project Wonderful network
• Example: You place an ad on a site that wants 4$ for 24 hours. You can stop the ad at any time and only pay for the time the ad was visible. If you stop it after 6 hours you will only pay 1$.

How does Project Wonderful work
Forum Thread about Project Wonderful (TIGSource Forums)

Using Project Wonderful To Advertise Indie Games (Studio Eres)

BuyAds.com

• Pay per day week/month or CPM model (depends on site)
• Fixed prices that vary greatly depending on the site you want to advertise on.
• Ad will be displayed on choosen sites from the BuyAds.com network
• Example: If the CPM of a site is 2$ it means that they charge 2$ for every 1.000 impressions (pageviews). So if you select 20.000 impressions you would have to pay 20$. In case your CTR would be 0.25% this would result in 50 visitors to your site.

BuySellAds

• Pay per month model or CPM
• Fixed prices that vary greatly depending on the sites you want to advertise on.
• Ad will be displayed on choosen sites from the BuySellAds Network
• Example: If the CPM of a site is 2$ it means that they charge 2$ for every 1.000 impressions (pageviews). So if you select 20.000 impressions you would have to pay 20$. In case your CTR would be 0.25% this would result in 50 visitors to your site.

Game Advertising Online

(site doesn’t offer pricing details upfront.)

Others

There are also Mochimedia, Facebook Ads, Google Adwords and Fusion Ads…

Moreover here is an Wikipedia Article about Advertising and you might find some inspiration for an ad idea over at Ads Of The World (it’s a site that showcases ads from around the world).