Free Windows tool No. 1: SpiderOak
At least a dozen different products will sync files across multiple computers, tablets, and phones. SpiderOak stands out for its “zero knowledge” approach to security. In short, there’s no way the folks at SpiderOak can look at your data: you, alone, hold the encryption key. SpiderOak offers several unique features, including automatic file versioning, with recoverable history. It allows you to set up restricted “Share Rooms” where only invited people can participate. Dropbox is a popular and powerful alternative, but not without its security concerns. Box and SugarSync are also worth a look, but if security’s your No. 1 concern, SpiderOak is the product to consider.