The Pepys' Diary site is currently republishing the diary as a blog, one entry per day, along with discussion. One thing that the discussion has brought to light is that there are other published diaries covering the same period, and they occasionally cover the same events: for instance, John Evelyn knew Pepys and attended many of the same events, so comparing his diaries with Pepys' allows the reader to see the same scene described from two different viewpoints. While The Diary of John Evelyn is available online, it doesn't take advantage of the wealth of annotations that have grown up around the better-known (and somewhat more readable) Pepys' Diary.
It would be interesting to have a site that collects together as many historical diaries as possible, so you could go to a date and see what Pepys, Evelyn, Ralph Josselin and any other contemporary journalist had to say about it, making use of historical annotations from all the sources. There's no reason this should be limited to the 17th century, since published journals exist from many historical periods. This could be generalised into a this-day-in-history site making use of other historical materials.