Data.gov.au - Is hosted by the Australian Government. At present it contains over 1000 datasets, including datasets on Construction materials, housing finance, planning, Property, and Transport. Contains a bunch of interesting apps and links to GIS software also.
Datacatalogs.org - Is a portal to open Datasets from around the world. API compatible.
Eurostat - Business and demographic statistics from the Eurozone. Also contains specialist datasets like rural development and land cover/use.
Statista - A wonderful data portal with an easy to use search interface. It contains information about the major construction contractors, up to date information about construction activity in the US, Europe and elsewhere around the world also. Number of hours spent by the construction workforce in the US per week - and much more. Well worth a look.
World Bank Data Catalog - A wealth of datasets from all parts of the globe. Contains World Development Indicators, World Bank Finances, Health, Education Stats Global Economic forecasts much much more! Largely API compatible.
There is also an interesting web page here containing 20 free GIS tools