I have had the best luck with an immersion blender but a food processor will work just as well. You may need to do a double batch though. It takes a few tries to get the hang of it!
Chipotle Mayo: add 2 tbsp adobo sauce to 1/2 cup mayo and blend well.
Garlic Mayo: add 2 crushed garlic cloves to 1 cup mayo and blend well.
Wasabi Mayo: add 1/2 tsp 100% pure wasabi powder to mayo recipe above.