Mare.
It took me 3 minutes to think that up.
Unfortunately, Dr_Paprika included it in his list a full seven minutes before you even started to think of it. 
Mum.
Short answer: Yes.
Shorter answer: Mo.
Mom. (If you’re into meditation)
I was thinking of the flower and of the phrase “mum’s the word”.
mum is short for crysanthamum (horrific spelling, I know). I doubt it’s a word in it’s own right.
Mum is very much a word, and a versatile word at that. AHD lists five distinct meanings for it. It’s a verb, adjective, noun, and interjection.
Munderwear.
yabob, how about doing it in one line (although not terribly efficiently):
$ for word in `grep '^m' /usr/dict/words`; do grep '^'`echo $word | cut -c2-`'$' /usr/dict/words; done
(note: my /usr/dict/words only has 57 such words)
Actually, if you want it relatively efficiently in one line, you can realize that comm takes “-” as a filename argument for stdin:
grep “^[mM]” words | cut -c2- | comm -12 words - | grep “…”
The final grep gets rid of single character and empty strings, in case your word list includes all single letters as “words”. If you like, you can just realize that “ma” and “mi” and so on ought to be included.
The UNIX system I just tried that on has 59. As I noted earlier, if your spell command is smart enough to stem rather than having “mutter” and “muttering” both in the list, for instance, you are going to get smaller results.
Mcheerios
Mdinosaur
Mmarmalade
Mmnemonic
Mlook Mpeople, Mhaven’t Mwe Mdone Mthis Mquite Menough, Malready? MmyMhead Mis Mspinning Mnow!
Do you consider “Mom” to be a word?
This seems to be more of a word game than a quest for a definitive factual answer, so I’ll move this thread to MPSIMS.
bibliophage
moderator GQ
In my neck of the woods, people from the Bay State are sometimes called “Massholes.” I would of course never say such a thing myself. Out loud. Where they can hear. Speaking of Massh–er, I mean, apropos of nothing, “manus” is a word.
I actually spent more time wondering if I could do this on my Windows machine (I killed my Linux box by “experimenting” too much; should be rebuilt by Monday (Gentoo)) than I did finding a few words…pfbob If I may ask, why exactly did you need to know this? Surely examples occured to you immediatly?