A friend just asked for hotel, restaurant and things-to-do recommendations for a weekender to London, as I'm from the UK - so I should know. Ok - so yes - I'm a brit, and I've been to London lots of times, but his question triggered me to think about the way I usually research spots to visit in foreign cities.
Before we get to that, my initial thoughts for London were the London Eye, Trafalgar Square, Covent Garden, Hyde Park and perhaps the Tower of London. There are also many fantastic museums and art galleries in London, but personally I prefer to spend more time outside than in. Would you add any others?
What to do
A solid suggestion I usually try when heading to a foreign city would be a free walking tour (free as in you pay what you think it is worth via a tip at the end) - as these generally give you an orientation of the city, and hit a few of the key spots in a couple of hours. In London the NewLondon Tour has good reviews on TripAdvisor.
The second source I check is always the TripAdvisor attractions lists, such as this one for London. The power of aggregated crowd-sourced reviews is usually a pretty good indication of quality. Wikitravel.org can also be a good resource for practical getting-around the city information.
Once on the ground, assuming I've got a couple of hours to kill. Personally I also love to use geocaching to find the best local spots to visit. Geocaching is a global activity where locals hide stuff in interesting locations, and then challenge you to discover it using the geocaching.com website and a hand-held GPS or smartphone. The fact that a local has hidden the cache in a location of interest usually means if you spend a morning geocaching in an area you'll discover all sorts of places and viewpoints that you might otherwise have missed.
To find inspirational photography locations - I'll check Google Maps with the photos layer turned on, Panoramio, 360cities new map or the same layers in Google Earth. I also sometimes checkout flickriver for interesting images posted to flickr.
A new service ShotHotSpot pulls highly rated geotagged images from flickr and panoramio to create a photographer optimized research tool.
Where to sleep and eat
In terms of accommodation - there are dozens of hotel search services these days that can help find the best value/location combination many of which I list on my travel page. My current favourite is hipmunk - which now has a superb map-based search engine that lets you search by ecstasy (which is a combination of price, amenities, and reviews).
So... did I hit the spot? Do you have any other excellent services that you use for travel planning?