Mac OS X only: Free utility OneWay puts an "Upload to" item in your right-click/Ctrl+click context menu for quick, easy uploads to commonly used FTP and SFTP servers.
Whether you've set up your own FTP server on a Windows machine, have enabled FTP sharing on your Mac (just go to System Preferences -> Sharing, then tick the File Sharing box to enable FTP), or you've got honest to goodness web servers you commonly upload files to, OneWay is a handy tool. Just create bookmarks for your favourite (S)FTP locations (including specific folders on a server); each bookmark you add will be added to your context menu. To quickly upload a file, just right-click/Ctrl+click it, select the server you want to upload it to, and that's all. OneWay is a free download for Mac OS X only.