This is going to sound a bit childish but a previous employer of mine used to do this on friday afternoons/evenings with surprising results, we had accountants mentioning that they picked up on details of things that manufacturing guys would be putting together, we gathered a lot of knowledge about people's area's of expertise. We even had a guy from accounting stand up and talk about his processes and procedures and how if other departments slightly changed how they did things then they could save everyone time in general.
Come 3pm on a friday there would be between 1 and 3 presenters, and between 15 and 30 people watching., people from around the entire company would gather around ( from sales, to manufacturing, to engineering, to accounts. ) Sometimes people would be prodded by their managers to be one of the first few. The topic was open, so you could talk about anything from current/past/future projects, sales opportunities at customer sites, inter/infra-departmental inefficiencies/efficiencies,
Sit around in the sales area, (because it has lots of chairs and benches to sit on as well as large whiteboards. ) A director comes down with a couple of slabs of beer. (for the non-australian's a slab is a 24 pack of beer.)
While presentations were going the beer supply was being replenished if it ran low, it was a extremely casual setting, you could ask questions during said presentations.
When the presentations finished sometimes people would do 'improptu' ones.
I would never stay past 7pm, but it I have heard that there were times when engineering and manufacturing guys would stay back past 10pm. both departments would normally close at 3:45/4:30 on those days.
Pretty much the deal was as long as someone was talking beer would be supplied and paid for by the company. And people were more than happy to stay back for hours unpaid to simply hang out and exchange information.