Scotland's People: Statutory Records Index (SRI), Original Parish Records OPRs) and International Geneological Index (IGI).
Scots Origins: Free International Geneological Index for Scotland.
FREECEN: Free Census search particulary good for Scotland 1841,1851 & 1861. Check the statistical page for availability of the enumeration district you want to search
The Highland Council Archives: Well worth a visit: Manual census and all other information search. See the web site for details of records, opening times etc.