One of the topics which came up in the MeeGo architecture discussion we reported on last week involved EDS (Evolution Data Server) versus Tracker. Likely because of that discussion, Philip Van Hoof has put together a performance comparision of the two for contact data storage: I've done a performance analysis and comparison of Tracker's RDF store versus E-D-S (Evolution Data Server).
Ove Kaaven recently released a Caller ID application for the N900, "callerid", which will automatically look up a phone number on the Internet and display who the number belongs to: It primarily uses XML configuration files to find out which web server to use for any particular country code and area code. Once the name has been looked up, it's displayed as a Hildon banner. (Yes, you need a working Internet connection for this to work. Also note that in many cases, 2G doesn't allow you to use voice and data simultaneously, so if you're using 2G and don't have WiFi, you may be in trouble...) The app can be found in the Extras-devel repository and comes with all of the standard warnings about using non-tested software; with extra sprinkles because of the integration with the telephony subsystem.