Combine the tomatoes, jalapeño, onion, cilantro, lime juice, salt, and pepper in a medium-size bowl.
Serve right away or refrigerate, covered, until ready to serve.
Notes
Most of the heat in a hot pepper resides in the seeds and white membrane inside, so if you remove all of those parts, you will just have the mild heat from the flesh of the pepper. Leave in some seeds for a bit more heat, and feel free to leave in all the seeds if you are looking for a super spicy salsa!