I don't remember exactly since I never bothered to use them myself (Wikipedia is enough for now while I finish my extensive central site content), but I do remember running across it about three months ago - I think it was Encyclopedia Britannica or some other recognizable name. The whole deal with the premium encyclopedias is that, yes, you can edit them like the Wikis, but the changes are only posted after they have been approved by a licensed moderator. They're desperate to find a way to compete. Only a fool or an old person would pay for a brand-name encyclopedia instead of an equally good and more dynamic free one that you can actually have a say in. It will be impossible to spam them but if your site is extremely relevant to the topic, you've got a decent chance I believe. I did not spend time researching this in-depth. You might see more of this trend in a year or two, also. If you can't find any encyclopedia like that since apparently this is not as common as I thought, then just forget about it for now.