drop.io: A gloriously clean and simple file-sharing tool. It's amazing that with all the solutions to this already out there, none have done it this well. If you need to share a too-big-to-email* file (or set of files) with someone, or share the same large-ish file(s) with multiple people, this is the best solution I've found. (Plus, knowledge of FTP soup: Not required.)
*For easy emailing of files, also see usend.io, built on top of drop.io.