Library & Archives Canada - I have used this site to find Canadian Census Records & Military Records but there is much more available. Although the last Federal administration slashed funding to these resources, a different party is leading the country now, so there is hope.
Archives Portal Europe - Searches all of UK & Europe for free, open genealogy archives.
Castle Garden - If you have immigrant ancestors who arrived in New York between 1855 and 1890, they were processed through Castle Garden and you may find them indexed here. Some records were destroyed in a fire, though.
Internet Archive Digital Library - there are all kinds of free resources here including passenger lists. I found two great-grandfathers on lists here when I failed to find them elsewhere.
The Brooklyn Daily Eagle archives used to be on this page but have been removed because they are no longer free. This newspaper is now accessible through newspapers.com.