I think hack days are great - sure, 24 hours in one space is an artificial constraint, but the sheer brilliance of the ideas and the ingenuity of the implementations is inspiring. They're a reminder that good projects don't need to take years and involve twenty circles of sign-off, even if that's the reality you face when you get back to the office.
I went because it tied in really well with some work projects (like the museum metadata mashup competition we're running later in the year or the attempt to get a critical mass of vaguely compatible museum data available for re-use) and stuff I'm interested in personally (like modern bluestocking, my project for this summer - let me know if you want to help, or just add inspiring women to freebase).
I'm also interested in creating something like a Dopplr for museums - you tell it what you're interested in, and when you go on a trip it makes you a map and list of stuff you could see while you're in that city.
Like: I like Picasso, Islamic miniatures, city museums, free wine at contemporary art gallery openings, [etc]; am inspired by early feminist history; love hearing about lived moments in local history of the area I'll be staying in; I'm going to Barcelona.
The 'list of cultural heritage stuff I like' could be drawn from stuff you've bookmarked, exhibitions you've attended (or reviewed) or stuff favourited in a meta-museum site.
(I don't know what you'd call this - it's like a personal butlr or concierge who knows both your interests and your destinations - curatr?)
The talks on RDFa (and the earlier talk on YQL at the National Maritime Museum) have inspired me to pick a 'good enough' protocol, implement it, and see if I can bring in links to similar objects in other museum collections. I need to think about the best way to document any mapping I do between taxonomies, ontologies, vocabularies (all the museumy 'ies') and different API functions or schemas, but I figure the museum API wiki is a good place to draft that. It's not going to happen instantly, but it's a good goal for 2009.
These are the last of my notes from the weekend's Open Hack London event, my notes from various talks are tagged openhacklondon.