DEF CON is one of the world's largest hacking conventions - it's beyond my description so if you're not already aware of it then do read the Wikipedia page and go browse some of the talks from last year, both audio and video.
For want of a better description the London chapter is called DC4420 - think international direct dial codes - and I've literally just returned from the basement of a nice pub just off Oxford Circus, with decent food, beer, and presentations.
Tonight we had a team of Quantum Cryptography developers from Geneva explaining the basics of the technology, incidentally giving a demonstration of how to "tap" a fibre optic network link - apparently much to the chagrin of Swiss bankers - using equipment to the total of around $1000; plus a Belgian demonstrating SystemTap for Linux, a instrumentation tool reminiscent of Dtrace and being demoed with common hackerly amusements like snooping terminal sessions, snooping application activity, or let's ban someone from opening any file whose name contains ".mp3".
It's free to attend - frequently with free beer, buying rounds being one of the punishments prescribed for egregious sales-pitching. If you turn up to the next one you will likely be round-tabled - the organisers push it as a networking opportunity - and possibly badgered into speaking at a future event.
And if you subsequently do speak there will be both trolling and heckling - not hostile, but pointed, and it is something of a tradition - but to survive you need only be both frank and honest; anything that's less than that will not go down well.
Two thumbs up.