Optical communication, in particular, wavelength-division multiplexing (WDM) technique, has become a promising networking choice to meet ever-increasing demands on bandwidth from emerging bandwidth-intensive computing/communication applications, such as data browsing in the world wide web, multimedia conferencing, e-commerce and video-on-demand services. As optics become a major networking media in all communications needs, optical interconnects will inevitably play an important role in interconnecting processors in parallel and distributed computing systems. In this paper, we consider cost-effective architecture designs of WDM optical interconnects for current and future generation parallel and distributed computing and communication systems. We first categorize WDM optical interconnects into two different connection models based on their intended applications: the wavelength-based model and the fiber-link-based model. Most of existing WDM optical interconnect architectures belong to the first category. However, in this paper we show that for applications that use the fiber-link-based model, network cost can be significantly reduced. We then present such a minimum cost architecture design for WDM optical interconnects under the fiber-link-based model. We also generalize the idea used in this design to WDM optical interconnects under the wavelength-based model, and obtain a new architecture design which can trade-off the cost of wavelength converters with the number of crosspoints in this type of WDM optical interconnect.