1/2 - 1 jalapeno1 (depending on heat of pepper and your taste), minced
1/4 medium red onion, diced (or green onion, shallot, etc.)
2 tbsp fresh cilantro, chopped
1 large clove garlic, crushed
Grate lime zest and juice lime. Combine zest, garlic, onion, chile and salt in a small bowl. Add cilantro and mix in thoroughly. Scoop avocado meat into the bowl, mash together with mixture using a fork or potato masher, leaving a few chunks of avocado. Add lime juice and taste, add salt as needed. Serve immediately.
Individual chile peppers can vary widely on heat, even within a species. I've had incredibly mild jalapenos and fiery hot jalapenos -- I've found the spiciest tend to be locally grown and mild tend to come from stores. But that's me.
No peppers? Try using some chile powder or cayenne.
Roasted peppers can also be very nice - try roasting an anaheim or poblano or pasilla pepper. How? Put it in the oven (or grill) and broil for 10 minutes (turning periodically) until the skin is blackened and blistered, remove to a paper sack for 10 minutes, then once it's cool enough to handle, peel off the papery skin, remove the seeds and mince the meat finely.