I love cayenne, don't get me wrong. It adds a fantastic heat to any dish from scrambled eggs to curry. It's just that sometimes I get a little bored of seeing "cayenne, optional" listed in recipes and need a reminder that there are other spices and seasonings out there that can give these dishes some serious kick.
I'll bet most of these seasonings are living in your fridge or cupboard right now, just waiting for to jump in and save the day. You might not think you can add Sriracha hot sauce to an Italian tomato sauce, but why not? It's just dinner! And by playing around with spices, you might discover a fantastic combination you never would have tried if you'd stuck to the recipe.
For adding heat to any dish, these are the spices and seasonings I turn to most often:
• Red Pepper Flakes - Good for more than just sprinkling on pizza, a pinch of these will infuse dishes with a sweet heat.
• Freshly-Cracked Black Pepper - Don't overlook pepper just because it's common; it's still good for a tingling back-of-the-throat spiciness.
• Hot Sauce - Tabasco, Sriracha, a bottle from the farmers market, I love them all. This is my favorite way of customizing my individual bowl if my dining companions want less (or more!) heat.
• Fresh Hot Peppers - These will be starting to come into season very soon. Look for regional and heirloom varieties.
• Dried Chile Peppers - Can't find 'em fresh? Try them dried!
• Chiles in Adobo - Mmm...smoky and spicy.
These are the biggies in my kitchen. What are your favorite ways to add spicy heat to a dish?
TW Salt Mill by Wil...

Harissa doesn't last long in my kitchen. I especially love it swirled into carrot soup. Or slathered over winter squash before it gets tossed in the oven. Or (my brother thinks I'm crazy for this) eaten straight from the jar.
I just bought a jar of powdered chipotle and will be using it just about everything for the next month!
Great topic! I often love a little kick in my food.
In addition to the ones you mention, some of my other favorites are ground white pepper, pickled jalapenos, ground mustard, horseradish, wasabi, aleppo pepper flakes, szechuan pepper, grains of paradise, and ginger.
I love how each contributes a different kind of heat.
Rep pepper flakes don't get used nearly enough! I use them in so much! I love the just right bit of heat they give.
FYI- I tried grinding them a bit in my mortar and pestle once and learned the hard way just the process of grinding them a second, you'll be sneezing constantly for 20 minutes. It was oddly hilarious.
Chile oil! Very easy to make at home, and great for stir-frys. I make mine from red pepper flakes, corn oil, sesame oil, fermented black beans, and a few other things. The recipe is originally from Barbara Tropp's China Moon cookbook.
You know what does a great job at warming you up and adds great wonderful depth too? Black cardamom! Or ground cloves. Like pepper but better. I love chillies but spices are fantastic for heat.
Sambal olek - firey delicious Indonesian chili sauce
I love Sriracha for straight up chili heat. Sambal olek and harissa are both great when you want some garlic flavor in there too.
I don't care for Tabasco. It's good for when you want your food to taste like vinegar, which is never in my case.
I'm with everyone else on red pepper flakes. I put them in and on just about everything. I enjoy spicy and while I can handle extreme spice, even the nice addition of red pepper is great.
Cayenne powder is also good but it creates a more back-of-the-throat burn I do not like as much.
Aleppo pepper has been a great discovery for me lately. I put it in everything from pasta to soup. Its not very spicy, but it adds a bit of a kick and a lot of flavor.
Aside from that, being Korean, I add a bit of Korean ground pepper for heat sometimes. Be careful, though, as it adds a LOT of heat.
Oh and the Portugese Piri Piri sauce in lieu of tabasco if you want that vinagery kick.
I use crushed red pepper flakes on everything! It spices things up by not using plain old black pepper all the time.
They are particularly delicious on crostini topped with fresh homemade ricotta, basil, good olive oil and fleur de sel. YUM!
I think Slow Lorus hit it on the head-- there are many different gradations of spice and heat. Everyone should experiment to find their favorites.
Don't discount other 'ethnic' hot sauces; there is a fantastic Jamaican Mango Hot Sauce that is quickly being drained in my house!