In case you haven't already seen my other posts about client wrappers, I work full-time developing modifications for programs such as the Minecraft client.
I am offering paid and/or ad-supported customized wrappers for hosts looking for that extra perk to help them stand out. It is essentially an in-browser Java applet that downloads and installs the official client from minecraft.net, then replaces bits of it during runtime to achieve a legally modified and distributed Minecraft client. You can offer this client on your own website, on any individualized sites you might offer, etc.
Licensing fees are negotiable, and expected to vary based on factors such as texture packs included, mods loaded, customized mods, and users. It's up to you if you want to pay for the license or display my advertisements.
For an example of an ad-supported version, see http://earth2me.com/minecraft/. It features average customization, without much in the way of documentation or branding. It is my "sandbox" version, where players who wish to try a beta version go to connect to their respective servers.