So, yesterday afternoon, Morten suggested it would be cool if there was a site that could score the days at Roskilde against personal preferences as expressed through Last.fm.
Indeed it would be, and since Morten does nice minimal interfaces and I do data gathering and mixing, we agreed to split the work, and build the Best day at Roskilde-finder.

It's worthwhile to have a look at what infrastructure we have used for this and which situational hacks are involved. I didn't have to scrape the concert program myself, as Steffen had already done that, through Yahoo's YQL.
What I needed to do is mine Last.fm's API for relevancy for those bands to merge with the user's favorite bands.
Present that to Morten's website as simply as possible and let Morten make a useful interface for the data.

It doesn't quite end there, though. Morten had previously exploited live play information from Danish National Radio to create a radio station persona on Last.fm.
Through Spotify, using Spotify's Last.fm integration he is also building a Roskilde Festival persona.
- these will give more general than personal answers: "If you're the kind of person listening to this radio station you will like".

It's interesting how much infrastructure is available - and useful - for a mashup like this.

We're using Yahoo, Last.fm, Danish Radio's website, Roskilde's website and Spotify as data sources/web services - and combining preexisting situational hacks from 3 people, on top of the obvious webservers and direct hacking.
These resources can be combined, and hidden away, in less than 10 hours to produce a coherent, simple and fun website.

Add instant distribution through Facebook and Twitter (Facebook wins) and there's a nice useful bit of mashup for an intended audience of 200-10000 people.