Programmable Web is a great resource for more information about Mashups and at the link has a GREAT summary of APIs available to mashup developers.
I'm very anxious to get going on my own mashup which is fairly simple but I think will be helpful and popular as part of my growing Highways.TV / Highways.com project. I'm taking the camera images from Caltrans and some other sources and mapping them to CA roadways along with weather information. Caltrans does this for some districts but not others, and confusingly Caltrans cams are sorted by district rather than roads. Districts may have some bureaucratic logic but travelers are not necessarily going to confine their interest in that fashion. Oregon's TripCheck system is a much stronger presentation of cam info than Caltrans'. Weatherbonk is doing a nice job integrating weather, maps, and some cam info but they are really short on traffic cams and the site seems to be clunky and slow in the display - though I'm not clear if they are simply suffering from popularity and can upgrade to speed things up or if the mashup itself is burdened by too many calls out to the data sources. I'm noting that Caltrans real time TV cams are hard to get to display and the lag time is so great even on a fast connection that it's not a very useful feature even when it does display.