Here is an idea that could have some potential - there has been discussion (by Peter R in particular) about tools for creating a "snapshot" of all UTXOs for a given date range (for usage in clone coins that will end up with the equivalent distribution that BTC had at the time of the snapshot).

Offering such a service (which requires a full block chain) could be a way to "earn some extra BTC by running a full node" (and perhaps there are other such services that could be useful - such as say statistical reports).