I've been using web databases to plan trips for more than a dozen years. Here are three of my favorites, two focusing on opera and concerts and a brand new one on London that covers all manner of events and venues, many of which are bound to be new to any but the most dogged and attentive traveler.
As I said in a recent post about attending a performance at the Opéra Royal in the Château de Versailles, two of my trusty tools for planning a vacation around music are Bachtrack and Operabase. The latter, as its name suggests, focuses solely on opera. Its coverage is international and its interface rather plain and old-fashioned -- which actually makes it a pleasure to use. A typical query -- all performances in Italy between 1 May and 15 September 2012, say -- yields a text-based tabular listing, with live links, that's easy on the eyes and the brain.
Bachtrack covers ballet and classical music as well as opera, and it does a fine job for most major European and U.S. cities. But Operabase is notably more comprehensive in its sphere of interest: it includes many more cities and indeed many more countries. Poland, for instance, and all of Latin America are part of Operabase but are missing from the Bachtrack database, as are some of the smaller, often interesting, provincial theaters in Europe -- at least for now: expansion is promised as more arts organizations become involved.
Yet it is a more up-to-date-looking site (containing ads, which I for one do not resent in the least: they're subsidizing a valuable service), and it has more features. You can, for instance, use its links to buy tickets for many of the performances it lists -- I just did this, and it worked to perfection -- and indeed to buy CDs of the music. Because the site is richer in information and doodads, the results of a query can spill over onto more than one screen, so you may need to use your browser's back button or make notes as you review the material.
The couple who developed and operate Bachtrack have recently launched what promises to be another terrific resource, but only for London: One Stop Arts. In addition to the London data that Bachtrack already contains, the new site covers museums, visual arts, kids' events and theater. I have been traveling to London regularly since 1970, and the listings of art and museum events are full of venues I've never heard of, let alone visited: The Alexander Fleming Laboratory Museum? A 1939 Modernist house in Hampstead operated by the National Trust? New ones on me. This is a very rich storehouse of travel ideas.
If readers know other similar sites, they're welcome to pass them along by adding a comment below.