"dbe" is the Double-Buffer Extension module. It double-buffers your display to prevent tearing/flickering (well... it won't fix a low refresh rate's flickering, but it helps with tearing). Basically, the X server draws to an off-screen buffer, then calls some sort of swapbuffers function that blasts the offscreen buffer into the framebuffer on the video card, all in one go. This happens during the vertical retrace, usually.
"type1" is support for Adobe Type1 fonts. That font format is a competitor to the TrueType font format, and not a bad one at that. If you look in /usr/X11R6/lib/X11/fonts/Type1, you'll see all your system's Type1 font files.
"speedo" is the same thing, but for Speedo fonts. These are in ..../fonts/Speedo.
"extmod" is a collection of very useful X extensions. Although I normally don't see it in a Load line -- I almost always see it in a SubSection line instead. Whatever. As long as this is working, I guess.
"freetype" is an opensource TrueType font renderer.
As for others, look in /usr/X11R6/lib/modules and /usr/X11R6/lib/modules/extensions to see what you've got available. Remove the "lib" and ".so" (or ".a") from those files' names, and you'll have your candidate for a module. However, looking at what I have available, there isn't much else that a lot of people can use...