adsense (google ads): adsense pays the most for as far as i know, their ads are also very clean and *often quite related so its a good sale (on the other hand it could cause your competitors to show their ads on your site, thanks to the great *relevancy of the ads)
cpx interactive (aka xtendmedia): cpx interactive and xtendmedia are 2 clones of the same thing. their ads are relatively safe (no viruses or spyware but quit a few redirects though!) they payout is crappy compared to adsense but awesome compared to, lets say adbrite.. also most ads from cpx interactive are borderline stupid, borderline porn.. probably 95% of their ads are basically advertising for low scams.. but at least they dont get your site blocked by google safe browsing like some others.. (in my experience cpx interactive pays a tiny bit better than xtendmedia and also xtendmedia forces you to send them annoying *invoices each month to get paid!! thats annoying cpx doesnt do that.. they just pay you without asking you to *do anything..) *note 10 october 2011: cpx has been extremely late of their payments (and answering emails) for the last few months, publishers are leaving left and right wondering if they will ever get paid, I do not recommend cpx anymore...
adbrite: they have about the same safety level than cpx interactive, generally safe but their ads are cleaner (less bikini girls, less advertising for scams) sadly their payout is quite bottom-low. if you want you can increase you earnings with them with these full page ads but thats at the price or seriously lowering your visitor's experience.
clicksor/adonion: trust me ive used these many times and their payout isnt too bad, but they always get your site blocked by google safe browsing sooner or later and then its *goodbye traffic for good... theire the most dangerous ad network ive ever seen. they will infect your visitors and kill your site its a guarantee.
cpx24: I registered with them. No need to wait for approval I placed their "banner" on a test page on my site (with nothing else on it) and reloaded a few times, about every 3 page loads I got redirected to unrelated crap. dont use unless you want to lose all your traffic very fast.
these ads will not get your site blocked by google safe browsing but it will still kill your traffic and get you *banned by lots of users and other sites that might have otherwise linked to you. on the other hand they generally pay well in exchange... good if your desperate for quick cash at the price of literally killing your traffic.
pinballpublishernetwork (used to be Zango): pops a gateway on your site asking ppl to install some crap to access the page (for free..) you can make the gateway be *forced or *optional (if you click cancel on install it will still continue to the page..) many ppl even though they are wrong consider zango to be malware... its wrong cos all the bull zango installs can be removed either from add/remove programs and/or your browsers add-ons menu.. but running zango for a while will get you a bad rep with anti-viruses companies like macaffe site advisor in my experience.. on the other hand its pretty good cash!
CPAlead: the good old *forced surveys.. everybody has seen them by now... this thing only works well when on *forced mode even though the *optional mode is available (zango performs pretty good in optional). in my experience, while its a bit like zango, it pays a bit less and while zango will give popups to your users (popups coming from the computer itself lol..) cpalead will give your users surprise fees on their cellphone bills!... so basically with cpalead you will be scamming ppl harder than with zango, for less cash... but it still works...