these slots are for proprietry network cards, unfortunately if your motherboard did not come with a card to fit the slot it is for all intents and purposes a useless waste of space because the cards are rarely sold on their own and are very expensive in comparison to a pci NIC.
From click here Communication and Networking Riser
Communication and Networking Riser (CNR), which was developed by Intel, is an open industry standard for a scalable riser card, which is a hardware device that plugs into a motherboard and holds chips for functions like modems and audio devices. The CNR architecture, and electrical, mechanical, and thermal requirements of the riser interface are defined in the specification. The specification was developed for products used to implement low-cost local area network (LAN), modem, and audio subsystems and supports broadband, multichannel audio, V.90 analog modem, Home PNA, and Ethernet-based networking, and can be expanded upon to meet the requirements of developing technologies, such as DSL. In addition to cost benefits, CNR has the capacity to minimize electrical noise interference, through physical separation of noise-sensitive elements from the motherboard's own communication systems.