Note the Diigo list with links to (free) Mathematics posters. See for example all the posters created by nrich of various problems on their site or the excellent classic mistakes posters. (For non Diigo users clicking on the green link will take you direct to the relevant website, select back to return to the list).
Note that the list is also available as an Evernote shared notebook. You do not need an Evernote account to use this, though it is excellent and it is free!
For vocabulary posters it is very quick to produce an attractive poster with a site such as Wordle for generating word clouds.
This post has proved to be very popular, an updated version can be found here,