Where can I download real time data for traffic and weather? I'd like to use it as an overlay on a mapping service of my choice (eg. Google maps, OpenLayers etc.)
Parse (using python/etc) RSS feeds from local authorities. Tough, but free.
It's a bit of work up front, but then you can write the data directly into your own feature classes, and reserve yourself. Kind of like ESRI's new geoEvent processor Server extension, but all custom through python. I update my services every hour - so far so good. I pull about 500 stations every run, and preprocess a feature class with rss urls/ids linked to points.
Note: You mention 'for the world' - yikes - you may want to select the top 50 cities or so to begin!
Make sure they don't change their format!