For a webplayer, if you host on facebook or your own site, you can use any number of online ad networks, but they won't be linked to the player like MochiMedia is for Flash games. You'll have to place banners, etc. with HTML like on any web page. You can also use offer-based networks like Offerpal and SuperRewards to monetize a game, where a user gets some kind of virtual good for filling out advertiser offers, but again, you need to host the game yourself, as these networks use php to communicate when a user has "earned" more goods for your game, which in turned can be used to dynamically update the variables in your game.

Note, though, that I've heard of complaints from the offer-based services due to delayed user payments and offers that results in spam and spyware.

And you can always upload your game to a portal, which will share their ad revenue with you, though until you get some serious traffic, you probably won't be able to quit your day job. Wooglie shares 50% and has a forgiving review process, though it's much smaller than some huge Flash portals. Extensive list of Unity-compliant portals in this question.

I've seen AdMob and Mobclix mentioned on the Unity forum, although I don't know how easy it is to integrate them.

you can not use most of the ad networks with unity's web player because it does not have a built in browser and most of ad networks display ads in HTML format

if you want to use them, you should parse strings yourself (i am not sure how it's possible) for example you can get an tag and then use it's href to download the image and then see if there is any tag arround it and open that if available when the users click on the image. but as you see it's not an easy stuff.

i think we should create tickets in feedback for in game browsers i think that they have engine size issues about it.

Here is my very easy mobclix plugin if anyone wants it

AndroidMobclixPlugin