Websonic noticed that Google's Gmail is testing ads below the email message, under the reply box. Typically Google has placed ads above the emails and on the right hand side, never on the bottom.
Here is a picture Websonic captured of an ad below the reply box within Gmail:
I personally do not use the Gmail interface all that often, but I do use it and I have never seen an ad in that spot. Have you?
Forum discussion at Google Blogoscoped Forums.
