I previously wrote about the potential loss of historical technical details as a result of the demise of Yahoo! GeoCities. I listed several useful technical pages hosted on GeoCities that might be lost. Others have also pointed out that GeoCities provided a perspective of the early days of personal web pages with profound value for those interested in studying the history of the web.
I read with interest Jennifer Van Grove's post One Man's Quest to Bring GeoCities Back from the Dead. The endeavor she highlights, instantiated as the ReoCities site, is interesting enough because of the salvaged data of potentially significant value. Perhaps even more interesting, though, is the technical challenge associated with this and the approach being used. More details on this can be found in The Making Of, a story of stress and scripting.
According to the Wikipedia entry for GeoCities, other efforts to archive potentially valuable data hosted on GeoCities include Internet Archive and InternetArchaeology.