
Python 3, 363 341 331 316 308 293 289274 characters
import os,sys,re,collections as C
c=C.Counter()
n=sys.argv[1]
o="graph d{"
for w in re.findall("[A-Za-z0-9]*""\w*",open(n).read()):c[w]+=1
del c['']
for w,x in c.most_common(75)[1:]:o+="%s[fontsize=%s,shape=none];"%(w,x/5)
open(n+'.dot','w').write(o+"}")
os.system("fdp -Tpng %s.dot>%s.png"%(n,n))
Call it like this: python cloud.py file.txt. The script uses Graphviz's fdp force-directed graph generator to generate the image (it will spit out a GraphViz file to file.txt.dot and a PNG image file to file.txt.png). This means you'll need to have Graphviz installed.
The image above is the cloud it makes of Shakespeare's Hamlet - sadlyyou can tell, since it doesn't seem to containcontains "To be or not to be"... There's also some delightful nonsense to be found:
It is, to You and I, the Ham of Not thatO lord have us, what now...
