Google Videos. After Google's acquisition of YouTube in 2006, Google Video seemed as redundant as a second navel. By 2009, the ability to upload new videos was shut down, although concerted protest by users kept Google from shutting the service off entirely so that any videos still there could be archived manually. Those who had spent money on Google Videos' download-to-own/-rent program found access to their purchased content gone, although those with outstanding credit in the system could have that transferred as funds to Google Checkout. (Later, Google announced it would also offer credit card refunds; in April 2011, it announced that it was keeping Google Video content up indefinitely, until all the remaining videos could be moved to YouTube.) As with some other closed Web services, the issue wasn't just the content but the existing user investment in the site, in multiple senses of the word "investment."
Sidekick. Back in October 2009, around 800,000 T-Mobile users who owned the Sidekick phone were in for a rude shock when the servers holding their personal data -- including email and contact information -- went down. It was originally reported that the data was lost for good, although the majority of the data was later restored. Not that it made for any less of a black eye for T-Mobile and Microsoft (which were managing the servers that kept the Sidekick data). Worse, users had no short-term recourse for recovery other than whatever data might have been synced to their computers.
The data service for Sidekick was discontinued for good on May 31, 2011. According to a statement by Microsoft, T-Mobile provided "an enhanced Web tool ... on myT-Mobile.com to easily export their personal data, including contacts, photos, calendar, notes, to-do lists, and bookmarks, from the Danger service to a new device, computer, or a designated e-mail account." If they had provided something so convenient during the earlier data outage, or as a routine way to allow Sidekick users to keep their data intact, the wailing and gnashing of teeth might not have been as loud.
Blogging and Web-hosting services. With blogging and free websites now throwaway commodity offerings, it's not surprising when these services bite the dust. GeoCities, an artifact of the Web's earliest commercial days, was widely lamented when its plug was pulled in 2009. Yahoo did little on its own to preserve the sites, but there were third-party efforts to save the contents of Geocities. Also, Windows Live Spaces was shut down in March 2011, ai which time users were given the option to migrate to WordPress. And as of May 24 this year, Yahoo's MyBlogLog was also canned; there are, however, tutorials on how to migrate your data from it.