The UK National Archives has released a new Global Search engine, which examines more than 28 million records from various archival repositories around the British Isles. It's quite a neat site, and I really like how they've done their subject headings - these were based on conservations with users, and it shows. The advanced search option allows searching by individual database, and seems very easy and adaptable.
Just in a few minutes of playing around on this site, I've already discovered some military records on my great-grandfather, who served in World War I; my family didn't know these records were extant. Quite exciting! Certainly try out the site, and use the feedback form at the bottom to let the Archives know what you think of it.