A Eurail Global Pass offers free travel for a certain number of days, but doesn't include seat reservations. Seat reservations can be made on raileurope.com (and their regional sister sites for Australia, Canada, and the rest of the world), but they charge a significant fee (can be around €10, often more than the reservation itself!) and can usually only send the reservation by mail, which is tricky if you're already travelling.
Where can I reserve seats to go with a Eurail pass, without the extra expense and inconvenience of booking through Rail Europe?