Visualizing large posets does somebody know if there is any software for visualizing very large posets? (like those in page 27 of this notes of Guenter Ziegler). They may arise (as in that text) by considering the face lattices of high dimensional polytopes. Thanks!
 A: John Stembridge's site containing Maple packages for symmetric functions, posets, root systems, and finite Coxeter groups could be helpful.
A: David Cook, Sonja Mapes and I have a package for Macaulay 2 which draws pictures of posets, either with or without node labels.  It will produce the TikZ code to include these in papers as well.
How large though is "very large"?  While there are a number of built-in enumerators in our package (lcm lattices, hyperplane arrangement lattices, noncrossing partition lattices, etc.) which can produce fairly large posets quickly, the only limitation might be how you're storing or inputting these posets.
A: I feel certain that Ziegler's Fig. 2.1 below (to which I assume you are referring?) 
was just drawn "by hand" 
in a generic drawing program,
such as Adobe Illustrator, or xfig.
          


          
(p.643 = 27th page of notes linked by Camilo)
This is probably not what you want, but Mathematica (via Combinatorica) can draw Hasse diagrams.
See, e.g., this link for how to accomplish this.

          
          

A: I'm seeing this question very late, but have two possibly useful suggestions for drawing posets:


*

*My favorite way to automatically draw large graphs with unknown structure is GraphViz.  See
http://www.graphviz.org/ 
With some work, this can be made to give poset diagrams.  That's one of the good options for output of Stembridge's Poset package for Maple, as mentioned in one of the other answers.  (But GraphViz is open source software, which seems like a significant advantage to me.)  Looking at the source code of Stembridge's package might be a good place to start in figuring out which GraphViz options to look at.

*GAP with the XGap (for Unix/Xwindows) or my own Gap.app (for Mac) front-end has code which allows manipulating Hasse diagrams of posets.  I have found it useful for making good diagrams of small to medium size posets.  I realize that the question asks about large posets, but include this for completeness. 
For good results, GAP requires you to specify the position of the vertices (and automatically draws the edges for you), so it helps if you have some idea of the structure of the poset.
(I can share an example or two of presenting poset diagrams with GAP, if it would be helpful.)
A: Curtis Greene et al. have a nice package at http://ww3.haverford.edu/math/cgreene.html.
