I recently came across the Museum of Online Museums (MoOM) site. The site is one of the initiatives undertaken by Coudal Partners, a company focused on design, web publishing, advertising, and commerce. Sadly the site isn't useful as a searchable database of museum exhibits and it is a bit awkward to navigate. However, it does provide an interesting look at a seemingly random collection of online "museum" exhibits. Note: Museum has been placed in quotes as the site uses the term museum very loosely and includes links to exhibits from personal collections.
The main feature of MoOM is an aggregate list site of museums with online exhibits and virtual exhibits. The Museum Campus portion of the site highlights some of the more well known museums (eg. Smithsonian, MoMA, and the Virtual Museum of Canada) which have an online presence. This list in an interesting mixture of institutions and it's not entirely clear what criteria an institution must meet to be placed on the list.
The site also includes a section devoted to interesting small collections and galleries. The majority of these exhibits are hosted on personal websites and are not affiliated with a heritage organization. For the most part this Galleries, Exhibits, and Shows portion of the site focuses on personal collections not on museums. Some of the more interesting collections currently in this section include: the Library of Dust, The Tiny Pineapple Nurse Book Collection, the Matchbook Registry, and the advertising gallery Found in Mom's Basement.
Although I didn't find the Museums of Online Museums site horribly useful from a heritage professional or educational standpoint, the site did provide an interesting look at what people outside of the heritage field consider to be museums or collections.