Merging wireless traces is a fundamental step in measurement-based studies involving multiple packet sniffers. Existing merging tools either require a wired infrastructure or are limited in their usability. We propose WiPal, an offline merging tool for IEEE 802.11 traces that has been designed to be efficient and simple to use. WiPal is flexible in the sense that it does not require any specific services, neither from monitors (like synchronization, access to a wired network, or embedding specific software) nor from its software environment (e.g., an SQL server). WiPal is available for download as free software. In this article, we present WiPal's design and algorithms. Most notably we propose a new synchronization algorithm that uses linear regressions on sliding windows of reference frames. A study of this algorithm shows it performs better than other synchronization algorithms. Experiments on real traces also show thatWiPal is an order of magnitude faster than other tools providing the same features.