JFuse

From O.O.Technology:

JFuse offers three main features. It can split a large file into smaller files, it can merge files into a larger file and provides an organizer. When files get posted on the web in many small parts JFuse can put them back together. This is often the case with movie files. JFuse will let you put parts 1-10 into the same file so it can be viewed without interuption. JFuse allows you to open movie files within the application so you can merge files and immediately view them without having to open a movie player. Splitting files is also a handy feature. If you want to attach files to an email, but your email provider only allows certain size files, JFuse will split them up into sizes you specify. If you wish to transport data via floppy disks, but the files are larger than the disk, JFuse offers the option of splitting files into floppy disk sized files.