For checking up on Multiple Listing Service data, I’ve been advised by agents in multiple states that the home estimate values can be WAAAY off (especially in transitional markets), but these are the commonly used ones: Zillow, Redfin, Trulia, Realtor.com. Still, I use these sites as a quick check and may often cross reference each site for the same property. I also use them to pull recently closed properties nearby for comps (within 6 months ago ideally, and within a 0.25 mile radius). They are great for basic data from taxes, insurance, bed / bath, square feet, and year built, including last pics from the previous rental listing or sale listing.

Craigslist. In some markets, people use this platform to post homes for rent (mom & pop owners). In combination with some other sites, it will give you a great idea of what rental comps are for your property.

Google’s mortgage calculator, built straight into the site. Helps you run numbers quickly (know this is only principal and interest, excluding any taxes and insurance).

Auction sites. can be used to find good deals, but I should caution you that you should really know what you are doing and know the market really well. Often times you will need to buy these sight-unseen.