In the PASS Summit Keynote (Day 1), they're demonstrating tools to join data from Excel, Marketplace, SQL Azure, using the "Data Explorer".
The thing that I'm liking about this is that you can specify that you want to do a lookup or a merge. Both joins, but two different types. I like this because when I write joins in T-SQL queries, I tend to have it in my head whether I'm expecting a Nested Loop (lookup) or a Merge Join. Seems Data Explorer lets me decide that right up front.
So you can pull data down from all kinds of different places, and publish it back out through the Marketplace. This means you can enhance other data sets, which other people can then enhance further, and this can continue to make some incredible data sources that can be consumed in OData just about anywhere.