"Banner" pay-per-click programs refer to pay-per-click ads as traditionally known.  Ads are displayed in a rectangle and, as a visitor clicks on them, the owner of the site gets a revenue-share of the revenue generated by the click.  Visitors do not have to buy anything for the website owner to be compensated.  Compensation is strictly based on the number of (valid) clicks generated. 

As this form of advertisement is easy to manipulate, it has generated a lot of criticisms due to click frauds.  Click frauds can come in many forms:

• A competitor of the advertiser can click of that advertiser's ads to increase his/ her advertising expenses.
• A website owner can artificially click on ads displayed on his/ her site to increase his/ her revenues. 
• Competitors of a website can click on that website's ads to "fake" the fact that the website owner is committing click fraud in order to have that owner banned from the pay-per-click program.  This latter situation is often invoked by webmasters banned from the Web's most popular pay-per-click program - Google's "Adsense."

The main pay-per-click programs are:

• Google's Adsense/ Adwords Program
• Yahoo's Publisher Network Program
• Chitika
• MSN's Adcenter
• Bidvertiser
• WidgetBucks
• Clicksor
• Quigo (AdSonar)
• Kanoodle

Depending on whether you view yourself as a publisher or an advertiser, the factors you will consider when choosing the appropriate pay-per-click program will differ:

• Pay-per-click for Publishers
• Pay-per-click for Advertisers

WidgetBucks is a pay-per-click program that differentiates itself from most other similar programs by its dynamic look.  As opposed to most pay-per-click programs that show text ads, WidgetBucks shows graphics and is pretty nice looking.  Another major claim made by WidgetBucks is that their CPM ratio is $3-$6 compared to $2 for most other traditional networks.

Chitika came up with Chitika Premium, a pay-per-click program that comes close to competing with Yahoo's Publisher Network.  In fact, it is debatable whether Chitika has surpassed YPN.  Many major websites have chosen Chitika, which is a recognition as to the quality of the offering.  One distinctive feature about Chitika Premium is that it only shows when someone is directed to a site through a search engine.  This means targeted ads and less spam for your site because repeat visitors will not see Chitika ads if they come to your site directly.

Bidvertiser is a second-tier pay-per-click program and a good alternative for webmasters who want to diversify their portfolio.

Google Adsense/ Adwords

The Google Adsense program is for publishers and the Google Adwords program is for advertisers.

Google Adsense is the Web's most popular pay-per-click program for publishers because it delivers targeted ads, which translates into higher conversion ratios.  Having targeted ads also helps to maintain the credibility of a site.  Google has a strict policy against click fraud and does not give second chances.  This means that once a publisher has been banned from the Adsense program, that publisher cannot reapply.

Google Adwords is also the Web's most popular pay-per-click program for advertisers.  One of its main weaknesses, however, is that Google has too many spam sites in its network, meaning that advertisers might sometimes see their ads associated with sites they would otherwise not want to be associated with.  To remedy this problem, advertisers can choose to advertise only on specific sites or to screen out sites on which they don't want their ads displayed.  Another problem with the Google Adwords program is that since it has a lot of advertisers and since its rate structure is based on an "auction" style, the more advertisers, the more expensive it gets (also keep in mind that there is no guarantee that the clicks generated will lead to sales).

Yahoo Publisher Network

The Yahoo Publisher Network ("YPN") is the second most popular pay-per-click program on the Web, but has come under a lot of criticisms lately due to the fact that the ads that are displayed are not targeted enough (a flaw in Yahoo's technology).  Another major downside for publishers is the fact that the YPN is only open to U.S. residents.  One good side about their network compared to Google is that they are more selective in terms of the publishers they admit to their program, meaning that there are much less spam sites that are part of the YPN. 

MSN Adcenter

MSN's Adcenter is currently open only to advertisers.  Ads are displayed within the MSN network of websites (i.e. the MSN portal and the MSN search engine).  Microsoft is in the process of setting up a program for publishers.