Here is an incomplete list of some software you might find interesting.... See the "Applications" and "Demos" menus too. Where can you find more software? Ask Archie.
You are probably using it. All documentation on-line.

www, lynx
Two terminal-based browsers for the World Wide Web. Lynx runs full screen on a vt100; www can run on a teletype. www can run non-interactively so you can use it in shell scripts.

wscrawl - Window Scrawl
A whiteboard drawing, painting, typing tool that can be shared over more than one X terminal across the internet. Your correspondents or collaborators will have to give you their DISPLAY variable to identify their screen and type xhost + to give you permission to access it.

Similar idea to wscrawl. Probably more versatile but messes up your color map

Predecessor of scrawl. Scribble on your root window or someone else's.

xmx, xtv
X-windows multiplexor: broadcast copies of your windows to remote servers on the net. Useful in long-distance collaborations. Both are buggy, have poor documentation.

xfig, xtexcad - Figure editor
Can make postscript figures suitable for papers, etc. Cheaper than a draftsman.

Image viewer (xv)
All purpose image viewer, manipulator, converter. The extremely lengthy, detailed documentation is here. The next time you put up an image, like this one, try resizing it and, to get more options, click the right mouse button anywhere over the picture.

Movie viewer (mpeg_play) New Motif movie player