A number of programming wikibooks, including books on assembly language, Ada, C++, and other languages have been listed at freeprogrammingresources.com. In almost every category, the wikibooks are listed at the very top of the page. Some examples:
x86 and MIPS Assembly books.
Ada Programming book
C++ Book (further down the page)
Not all of these books are very complete, but they are still good resources and can use all the publicity that they can get.