HJSplit is a popular freeware program to split and recombine files. The program is available on Windows, Linux and a variety of other platforms.
Why should one want to split and recombine files? For instance, think of a file of 50 Mb, and try to send it to a friend, post it in a newsgroup or upload it to a Website or FTP server. It is usually easier to send/receive, upload/download smaller parts than handle the entire file in one go.
HJSplit can also be useful for backups. A file of e.g. 10 gigabytes in size can be split into smaller parts which then can be burned to CD's, DVDs, copied to USB sticks or uploaded to an online backup service.
HJSplit for Windows and Linux support file sizes of over 100 Gigabytes, MD5 checksums, file-compare, "run without install" and both editions are fully portable.