A quick and dirty mortgage simulator that I created back when I needed to get a mortgage. Still use it from time to time to figure out next year's monthly premiums, the interest paid throughout the past year, tax benefits, etc. It's kinda customized for the types of mortgages available in the Netherlands and their taxation rates and benefits though, so probably not readily usable for other countries.
I won't claim it's 100% accurate either because it's not. It just barely covers the basic taxations property owners are subject to in NL, and the regulations change pretty much every year, so any calculations made by it should be taken with a grain of salt.