TV Tropes website.
In its own words: Tropes
are devices and conventions that a writer can reasonably rely on as
being present in the audience members' minds and expectations. On the
whole, tropes are not clichés.
The word clichéd means "stereotyped and trite." In other words, dull
and uninteresting. We are not looking for dull and uninteresting
entries. We are here to recognize tropes and play with them, not to
make fun of them.
There are so many tropes listed on the site that it can be a bit daunting, but generally I'll start with the genre or topic tropes and just have a hunt until I find something that looks interesting.
Here's an example - I'm writing a sci-fi story based on Earth. The characters are part of the space programme and are building a ship that will begin the colonisation of Mars. But now I've run out of ideas! Go to TV tropes, look under speculative fiction and read down the list. There are 1,208 tropes listed under spec fiction, so something is sure to jump out. In this case, let's go for Wetware Body, Absent Aliens and Asteroid Miners. Inspiration found, the story can go on!
As an aside, a couple of years ago I met someone doing NaNoWriMo who was basing her entire novel on tropes. It sounded like an interesting idea.
Have you ever deliberately included tropes in your stories? What are your favourite tropes?