it's ironic because technically Yahoo and their acquisitions are sound. Flickr is still one of the best looking photo sites around, and from what I hear from my Photography friends performs like a champ. Tumblr likewise is a good technological investment. I think the failure has been turning good technology into money, which is where a lot of companies fail. The beauty of Flickr and Tumblr are ruined as soon as you start heaping crap tons of banner ads on every page. But that said you still have to find a way to generate revenue from those investments.
A couple of others that stand out: GeoCities, Zimbra (Yahoo bought it for $350 Mil, sold it to VMware for less who sold it down the road as well), etc.