More formal:
* Stutter: This is the most common synonym and is generally interchangeable with stammer.
* Hesitate: This emphasizes the delay in speech, rather than the repetition or broken words.
* Falter: This suggests a loss of confidence or fluency in speech.
* Stumble: This implies a momentary interruption or difficulty in speaking.
* Dysfluency: This is a more clinical term for speech difficulties, including stuttering and stammering.
More informal:
* Spit out: This suggests a hurried and unclear delivery of words.
* Mumble: This implies unclear speech, often due to nervousness or shyness.
* Fumble for words: This suggests difficulty finding the right words to express oneself.
* Have a brain fart: This is a more humorous way to describe a momentary lapse in speech.
The best word to use will depend on the specific context and the level of formality you want to achieve.