I’ll focus on analogue sites for Mars habitability for the search for life on Mars as that’s a special interest of mine. For temperature and aridity, our best analogues are the Atacama desert core and the McMurdo dry valleys in Antarctica.

Researchers scout out field sites in Antarctica's Beacon Valley, one of the most Mars-like places on Earth. Image credit: NASA

It’s obviously not at all exact, but the closest we have in some respects.

Don Juan pond in Antarctica has the saltiest naturally occurring permanent water on Earth, useful for studying the limits of range of habitability of water on Earth / Mars.

The bacteria from Blood falls

metabolize sulfate and ferric ions. It’s a good analogue for Martian glaciers, if any of them have subsurface water, e.g. through hydrothermal heating.

Rio Tinto, Spain

This is the largest known sulfide deposit in the world, the Iberian Pyrite Belt

It may be a good analogue of some subsurface environments on Mars

Many of the extremophiles that live in these deposits are thought to survive independently of the sun. It's rich in iron and sulfur minerals such as

Jarosite on quartz Potassium iron sulfate Arabia District, Pershing County, Nevada 2779

• hemtite (Fe2O3) which is common in the Meridiani Planum area of Mars explored by Opportunity and though to be signs of ancient hot springs on Mars.
• jarosite (KFe3+3(OH)6(SO4)2), discovered on Mars by Opportunity and on Earth forms either in acid mine drainage, during oxidation of sulphide minerals2, and during alteration of volcanic rocks by acidic, sulphur-rich fluids near volcanic vents[139]

The fumaroles of mount Erebus might be an analogue of Mars - if Mars had ice fumaroles they would be hard to spot from orbit and could be a way for it to be geologically active yet not spotted from orbit.

Also an analogue of past sites on Mars, Home Plate on Mars, explored by Spirit is thought to be an example of an ancient fumarole on Mars.

Lechuguilla Cave

is a possible analogue for sulfur based ecosystems underground on Mars if they exist.

Our basalt lava tube caves are an analogue of Mars lava tube caves.

Then you have the magnesium sulfate lakes, “soda lakes”

Opportunity found evidence for magnesium sulfates on Mars (one form of it is epsomite, or "Epsom salts"), in 2004. Curiosity has detected calcium sulfates on Mars Orbital maps also suggest that hydrated sulfates may be common on Mars

Spotted Lake in British Columbia in Canada. Its sulfate concentrations are amongst the highest in the world. Every summer the water evaporated to form this pattern of interconnected brine pools separated by salt crusts

Then subglacial lakes like lake Vostok may give analogues of habitats below the surface of the Mars ice sheets if they exist.

If the ice sheets are deep enough on Mars, depth of 4–6 kilometers, the ice is so insulating that the base will melt through normal geothermal heating, not of a hot spot, just of the crust itself. However the martian north polar ice caps is probably only 3.4 to 4.2 km in thickness. It might not be deep enough for the ice to melt - but if the ice melted for some other reasons, then it could remain stable as a liquid lake at any depth over 900 meters.

For more details and the citations, as well as several other Earth analogues for the present day habitability of Mars, see my: Places on Mars to Look for Microbes, Lichens, ... also available on kindle as Places on Mars to look for Microbes, Lichens, ... Salty Seeps, Melt Water Under Clear Polar Ice, Ice Fumaroles, Dune Bioreactors, ...: Where early Mars lifeforms could survive to the present day