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Any wiki, most famously wikipedia, is a site that allows anybody to log in and update pages. This allows you to enter data as it becomes available, easy to update at any time.
There are many online genealogy sites that allow you to do much the same, the best known are Ancestry.com (which has some nice features like census page look-ups, but to make use of most features you need a rather pricey membership) and FamilySearch.com (run by the Latter-Day Saints, so it's free). FamilySearch uses a wiki format and encourages you to connect up to existing family trees already entered by other members. The problem is sometimes you find an individual already in 2 or more trees, with slightly differing details (number of siblings, birth date, etc). You can merge the individuals if you're sure it's really the same person, but you're kind of trampling on other people's data at that point.
By having my own wiki site, first, it's specific to the Ungerecht family. Secondly, I can make sure no bad data is entered (although I anticipate I will be the primary author, at least for some time). And as an important bonus, there is a PDF export feature, so the whole site can be exported at any time with the latest data as an Ungerecht Family Tree booklet.
Anybody can edit these pages, it is similar to Wikipedia (it is a DokuWiki, if you're interested). This page describes the syntax, or contact me if you want me to add information for you.
Additional pages may be created as needed (you must register first). Clicking on the pencil on the right hand side will give you the option to edit or create pages. For new family pages, I am starting the link (and page name) with the year and underscore. This allows the PDF output to be sorted by year.
Since this is accessible to anybody on the internet, I am not showing the birth year for people born after 1950, and I am not showing any person born after 1980. I can add whatever is desired to a GEDCOM that can be distributed privately among family members.