We present a novel system that helps nonexperts find sets of similar words. The user begins by specifying one or more seed words. The system then iteratively suggests a series of candidate words, which the user can either accept or reject. Current techniques for this task typically bootstrap a classifier based on a fixed seed set. In contrast, our system involves the user throughout the labeling process, using active learning to intelligently explore the space of similar words. In particular, our system can take advantage of negative examples provided by the user. Our system combines multiple preexisting sources of similarity data (a standard thesaurus, WordNet, contextual similarity), enabling it to capture many types of similarity groups ("synonyms of crash," "types of car," etc.). We evaluate on a hand-labeled evaluation set; our system improves over a strong baseline by 36%.