MIXRANK- A good tool for doing Competitive Analysis

Step 1: Pick a Competitor to Research
MixRank makes is super easy to get started. Just start typing in a domain name and you'll see a suggested list of names along with the amount of ads available:

MixRank breaks their tool down into 2 core parts:
Ads (text and display)
Traffic Sources

Step 2: Working with Ad Data (Text and Display)

Let's start with text ad options. So with text ads you have 3 areas to look at:
Active Ads
Ad Reach
Best Performers 

The image above is for "Active Ads". In the active ads tab you'll get the following data points (all sortable):
■Publishers - maximum number of AdSense publishers running that particular ad
■Last Seen - last known date the ad was seen by MixRank
■Frequency - amount of publisher sites on which the ad appeared
■Avg. Position - average position of the ad inside AdSense blocks
 

If the advertiser is running Banner Ads you can see those as well:

When you click on a banner ad you'll see this:

Step 3: Traffic Sources

Now that you have an idea of what type of text ads and banner ads are effective for your competition, it's time to move into what sites are likely the most profitable to advertise on.
MixRank gives you the following options with traffic sources:
Traffic Sources - domains being advertised on, last date when the ad was seen, average ad position and number of days seen over the last month
Reach - total number of publishers the advertiser is running ads on

 The traffic sources tab shows:
Uniques - estimated number of unique visitors based on search traffic estimates
Last Seen - last date MixRank saw the ad
Days Seen - number of days over the last month MixRank saw the ad
Average Position - average position in the AdSense Block