Search API: used to be able to submit keywords and get XML results back, now it's AJAX-only and spits out formatted HTML, only 10 results, and different from real search results. Mostly useful for people involved in SEO. No big loss without it, but just means you have to scrape SERPs instead of getting them nicely bundled up.
Desktop: I used it for a while, then found Everything (http://www.voidtools.com/) which is far faster and more responsive.
Bookmarks: Edit: It is still there.
Health: I didn't use this directly, but many of us in the healthcare industry were looking to see how Google could shake things up. Apparently, PHI is hard and Google discontinued their service.
Voice: It hasn't been discontinued yet, but I feel certain this will be the next big product to go. The service (and app) hasn't been updated in years, there is no clear monetization strategy aside from international VOIP calling (which other applications do better), and the messaging portion is mostly covered by cellular carriers as "visual voicemail" with the widespread use of smartphones.