Habitable Exoplanets Catalog (HEC) and at The Habitable Zone Gallery is testament to the fervor at which scientists are focusing on finding that sweet spot.
All these are excellent! And now somehow i think that there should be an additional field of Thought to consider when trying to find life in other places.
How about the idea of Habitable Bubbles? It's very simple. Habitable Bubbles are tiny packets, or "bubbles" of environment where life can thrive. In contrast with the Habitable Zone, which is often represented as a disc around stars where liquid water can exist, Habitable Bubbles can simply be represented as tiny bubbles scattered throughout a planet--on the surface and underneath, and even above it, in the atmosphere.
The idea of habitable bubbles can complement the Habitable Zone, and their relationship can be explored in this way: a planet that orbits within the habitable zone may probably have more habitable bubbles than a planet that lies outside the Habitable Zone. Of course, that is not the rule, but it's something to think about, for starters.
In essence, Earth is a habitable bubble in and of itself. And it is a bubble relative to a 'hostile' environment that is outer space. It is pretty obvious, but this simple idea of a habitable bubble can be scaled to other astronomical objects and not just planets. If Europa is teeming with hydrothermal vents, then Europa has a lot of potential habitable bubbles within that icy crust. Now think of exomoons and how many more there are elsewhere.
The point is this: even planets that lie outside the Habitable Zone can have habitable bubbles. In these 'extreme' planets, where a large part of the planet is considered unfriendly to life, there may still exist small packets that may be conducive to life.
I think that viewing habitability in terms of 'bubbles' is a good addition to our methods in thinking about life. A planet that may seem hostile at first sight could in fact harbor habitable bubble regions that are hidden.
Perhaps thinking in this bubble paradigm can provide additional insights about habitability elsewhere in the universe. For example, Hydrothermal vents can generate a habitable bubble, the heat from black smokers create a sphere of environment that can support life amidst the frigid waters of the antartic.
The young earth itself was once an inhospitable planet. Yet somewhere, somehow, a tiny region of Habitable Bubble provided an opportunity for that spark of life. And the subsequent interaction of the bubble and life itself gradually expanded the sphere of habitability until the whole Earth itself became habitable.
In the search for life, perhaps we should not only search for planets within a habitable zone, but we should also be on the lookout for planets with potential Habitable Bubbles!