• Free 1 : Kinda like the concept often used by Telecom operators. Buy a subscription and get the mobile phone for free. (Too bad this concept isn’t allowed by law in Belgium…. “Koppelverkoop”)
• Free 2 : The concept where the whole web 2.0 is running towards… Offer something for free to your community, but let an external party pay for their “subscription” by means of advertising.
• Free 3 : Shareware meets web 2.0… Get a limited version for free, but pay for the premium version.

You might also want to read the original article…

• They take care of the advertisers. Which enables you to have more time to work on your website, or whatever…
• The money generated from all the advertisers comes as a bundled payment. This is more friendly on the expenses of your (bank) transactions.
• Most (if not all) ad networks provide a clean interface where you can see the statistics about your sites & ad programs.
• Networks want their publishers to generate revenue (as they take commision), so they are (mostly) very helpful in supporting your stake.

But also disadvantages:

• Split revenue : the also take a piece of the action
• No direct contact with your advertiser.

Not all AdNetworks are friendly for an European publisher. Some networks only pay by “check”, this has it’s disadvantages:

• checks can get “lost” in the mail, so they have to be resent, and that brings an extra costs
• cashing in abroad checks mostly means high costs provisioned by your bank

So the networks who provide payments by paypal or wire transfer are more interesting…

There are quite a lot of ad networks out there, but let’s start with the networks I’m accustomed to:
TradeDoubler.com
I got referred to them by a friend, and I’ve kinda stuck with them for certain niche sites. They have a lot of partners/advertisers, which broadens your options a lot. They are very interesting for European publishers, as they pay thru wire transfer. And they have a very interesting minimum payout of 50€, where most networks use a minimum of 100€.
Some partners they use: eBay, AppleStore, iTunes, Dell, Jamba, Stepstone, … etc

Google Adsense
Probably the most know ad network. Their flagship are the contextual ads that you’ve most certainly have come spotted on a lot of sites.
Example :
The reasons I mainly use them is because they are easy to blend in and not region limited. Yet their minimum payout threshold is $100. And you have less control about the type (read: payment rate) of the ads that will be shown. They show non-paying public-service ads when no others are available (this can be disabled though), but you don’t know which payment rates will be used. Okay, they use the highest paying first, but the control freak I ‘m kinda hates the idea…
NOTE: Google is -very- strict on their policies… a good friend of mine runs a wallpaper site, and he got removed from the program (after nearing his payment threshold) for invalid content (he didn’t own the copyright to the photos he used in the wallpapers).

Other available AdNetworks, to make the post complete :

• Kontera : They have a nice inline contextual ad system.
• TextLinkAds : A system where people can bid what they want to pay (monthly) for showing their ads on your site. Sounds good, but only useful if you have a decent pagerank or alexa rating. Haven’t used them myself, but the friend with the wallpaper site is quite fond of them. (UPDATE: trying them out myself, as my friend is very happy with them!)
• AdBrite : By default you only get a “If you want to advertise here, click here”-link… Where’s the added value in that? My suggestion: stay clear of this one, various forums indicate that revenue generated thru their program isn’t great.
• ClickBank : An affiliate model system, it might be interesting, but haven’t had the chance to check it out.
• Consumption Junction : If you want to work with eBay, use Consumption Junction, and not TradeDoubler, as the revenue model with consumption junction is FAR better.
• Azoogle: a lot of different ad models, I’m currently applying to them, so I can’t give you the heads up on them
• Chitika : Got refused for their program, on grounds of not being big enough… You might be more lucky than me.
• Yahoo : The main competitor of Google Adsense.
• Amazon : Affiliate model, but it’s said their commision isn’t ideal (4% orso).
• Maxbounty : Affiliate model, kinda like tradedoubler, but a gui that looks a bit less.

If you’re considering using multiple ad networks, try using OpenAds. A bit more info can be found in an article that a wrote a while back.

Good luck generating revenue!