For Nova Scotia, it's the Nova Scotia Archives (NSARM) - their vital statistics search page is a great way to find digital copies of birth, marriage and death records. You have to play with the search settings and try different spellings to get the right records but otherwise it's a fantastic tool.
For New Brunswick, there's a similar tool available on the Provincial Archives (PANB) website. You can also download digital copies, and the marriage records are available to a much later date than Nova Scotia.
Prince Edward Island doesn't have the same options available for genealogists. The Provincial Archives (PARO) website offers a search tool but it only returns transcripts and indices rather than digital copies of the originals like NS and NB. It's a good finding aid but you have to either order copies from them or refer to other sources for more complete information.
Prince Edward Island researchers can also take advantage of a great website called The Island Register. It contains myriad resources for PEI and links to many others. Unfortunately, the owner of the website passed away in 2021. His family is maintaining the website for now but will not be performing any updates.
Automated Genealogy is a great finding aid for people in the 1901 and 1911 Canadian census enumerations, and it links back to the original census pages at Library and Archives Canada.
For all of the provinces, there are provincial and county GenWeb projects. Unfortunately, while many of them are still available, I'm not sure they are all maintained or updated on a regular basis. Still, they sometimes contain information not found elsewhere, which means they are worth checking out.
You can find links to all of these sites on my Links