I have a raster layer with travel times from a specified point in my city's downtown. What I'd like to do is select all of the census blocks that lie at least partially within 45 minutes of downtown (i.e. I'd like to select all of the vector features that contain at least one raster pixel with a value of less than 45). I'd like to do this in QGIS if at all possible; suggestions for other FOSS software are also welcome.
This is but one possible way to do such an analysis using QGIS. Spatial databases such as rasterlite or postgis is another option.