When he isn't telling bad jokes Chris can be prodigiously creative, and I'm fairly sure Pronto was his idea originally. We wanted to combine faceted search with autocomplete, and so had the idea to help users complete sentences from a controlled vocabulary like "I want a hotel in Paris on Friday". No clever natural language processing required, and across our whole site (including "advanced search") it combined something like 160 separate input fields into one.
It took a lot of tearing hair out during something like 11 rounds of user testing until we got it working nicely (if you've worked on this kind of interface I'm sure you'll be familiar with the users who look resolutely down at the keyboard and hammer out their sentence in full before looking expectantly up at the screen, during which time every clever bit of UI has been and gone already).
There was a mobile version (with some help from Ribot) that worked nicely, a trademark ("pronto" had nice connotations of speed and worked across Europe) an embeddable verson for affiliates, great little maps and photos that slid in as you typed (lots of great UI polish from Sam) and given we worked in a pan-European role we also did this in French and German (with some trepidation about German sentence structure, but we got there). Gradually through this process we realised we could mess around with the words quite a lot, from Chris' original Easter eggs (getting better deals if you add "please" to your query), to an explicit marketing campaign (which I think may have been Cathy's idea) of doing regional UK accents, so we ended up with "Howay man! Aa’d leik te gan bi plane te Barcelona, pet". And then we had some further Easter eggs with a very posh version and one where you could endlessly add "yeah but", "no but", "innit".
Unfortunately it isn't running anywhere any more as far as I know and the original video demos have been taken down - perhaps all that's left is this tiny Pronto demo on vimeo.