The explosive growth of single-molecule techniques is transforming our understanding of biology, helping to develop new physics inspired by emergent biological processes, and leading to emerging areas of nanotechnology. Key biological and chemical processes can now be probed with new levels of detail, one molecule at a time, from the nanoscopic dynamics of nature's molecular machines to an ever-expanding range of exciting applications across multiple length and time scales. Their common feature is an ability to render the underlying distribution of molecular properties that ensemble averaging masks and to reveal new insights into complex systems containing spatial and temporal heterogeneity. Small fluorescent probes are among the most adaptable and versatile for single-molecule sensing applications because they provide high signal-to-noise ratios combined with excellent specificity of labeling when chemically attached to target biomolecules or embedded within a host material. In this review, we examine recent advances in probe designs, their utility, and applications and provide a practical guide to their use, focusing on the single-molecule detection of nucleic acids, proteins, carbohydrates, and membrane dynamics. We also present key challenges that must be overcome to perform successful single-molecule experiments, including probe conjugation strategies, identify tradeoffs and limitations for each probe design, showcase emerging applications, and discuss exciting future directions for the community.