I confess to serial naughtiness when it comes to US purchases (I practically still live there, or so it seems) - savings certainly not as good as the $2=£1 days, but a lot of companies still do the dollar=pound equivalency (Apple, I'm looking at you) which can still make for a good saving. With electronics, it's usually just case of swapping the power lead for proper chunky UK electricity. Most warranties seem to be global these days, I've never had an issue. J&R and H&B are NYC essentials for cameras and electronics.
Clothes and shoes are also generally cheaper.
As mentioned, watch for sales tax, since quoted prices won't include it. Runs at 8.875% in NYC, which is one of the highest rates outside of California. Some states levy no or differential rates on clothing. There are a few states with no sales tax (New Hampshire, Alaska, Montana, Oregon, and Delaware) - things aren't always that cheap though, for instance in Delaware the state taxes the business direct on their receipts, and so they pass it on via the sale price of an article. Still, it's always a lot less than 20%.