For Scottish ancestors, we used to recommend the Scots Origins site http://www.origins.net - we have had a tremendous amount of good information from it at very moderate cost. One problem used to be that there is no information less than 100 years old on the site (because of the 100-year disclosure of information rule)
It used to be possible to sign-up for Origins.net News which generated some 20-30 E-mails per day discussing useful sources of information.
Very sadly, in September 2002 this valuable source of information ceased. Its function reverted to the GROS (General Register Office for Scotland). So far they done nothing of use to you or me! There is a statement that there will be a site providing the information, estimated at "the end of 2003" and it will contain the 1901 and 1891 census information. We will have to wait and see what happens. It may be that the site will be ancestralscotland.com (this already exists and may give us a link to the data, if and when it appears)
Much of the 1901 Census is available at www.1901census.nationalarchives.gov.uk but does not cover Scotland. For Scotland use the link www.scotlandspeople.gov.uk
We have also tried the Mormon site http://www.familysearch.org in the USA, but that was of no help in our case.
The following sites have been very useful...
For general UK searches a marvellous reference point is the British 1881 Census with details of all 30 million people on it, including their address and occupation. Quite a few libraries have this set of CDs and quite a few sets are privately-owned, since they only cost £30. The information comes on 25 CD ROMS and are sold by the Mormon Church, the address in the UK for purchase is....
Family History CDs
399 Garretts Green Lane
Birmingham B33 0UH
Tel: 08 700 102 051
Good luck with your searches.