UUENCODE/UUDECODE
This program allows you to encode a binary file in such a way that it can be safely transmitted over a text only channel like e-mail. Conversely, it lets you decode those files that look like gibberish and that have been uuencoded at the other end. The basic UUencode/decode and XXencode/decode functionalities are freeware; the MIME option is shareware, $10.
MPACK
Same stuff as UUencode and decode, except that it also does the MIME base-64 format. Freeware.
PKZIP/PKUNZIP
This is one of the most popular DOS archiver/compressor programs; you'll see that most of the PC programs available for download (except this one, of course, for obvious bootstrapping reasons) are distributed as ZIP archives. The ZIP format is supported on many other platforms too. Shareware.
LHA
This is another archiver/compressor, not as popular as ZIP but completely freeware. The author, Haruyasu Yoshizaki, even released the source as freeware! Use it to decompress files that end in .LZH.
ARJ
Another shareware archiver/compressor. Use it on files that end in .ARJ.
F-PROT
One of the best antivirus programs, regularly updated. Get the file called fp-*.zip: the file name changes as new versions come out; also, look for more antivirus programs in that same directory. You can also get it from this other place, which may be slower but may carry a newer version. Use it to check anything you receive from external sources! Shareware, but free for personal use.
LIST
Possibly the best DOS text viewer. Accepts files of ridiculously large size and has more options than you can imagine, but it's still lightweight, straightforward and fast.
PGP
Pretty Good Privacy, the powerful public key cryptography program that lets you keep your email secure. Freeware, source available. United States users please be sure to only download from a USA site to comply with your country's restrictions.
VDE
I'm not promoting this one as my favourite DOS text editor (I've only test-driven it for a few minutes) but it's an adequate replacement if you have a version of DOS that doesn't include EDIT.COM or if you need to do amazing things like (drum roll...) word wrapping, which are beyond EDIT's capabilities. It uses the wordstar command set, has menus (F10 to get them), lets you select files with the arrows instead of necessarily typing a filename. And it runs on small computers too.
6x13
This is the 6x13 font from the X Windows distribution, in a format suitable for Windows. It is one of the most legible monospaced bitmap fonts and it works beautifully for source code. There is a standard version and a useless bold version too.