Most desktop publishing types making posters, pamphlets, and newsletters will gravitate toward the largest, shiniest apple on the proverbial tree, most notably Adobe InDesign (there's a free trial of the $700 app for the curious).
However, the smaller, more muted Scribus (for Mac and Windows) or Portable Scribus (read review) could be a riper pick for you. Being able to tote a full program on a USB drive is especially beneficial for students, small businesses, and locale-shifters; amateur users will find more than enough features to create good-looking documents.
Some features are a little rough around the edges. I'm thinking specifically of the text editing tool, which requires going into a story editor to format and doesn't even offer a preview. I much prefer to edit my text on the screen, sliding the mouse to an extra toolbar to change font type and size, add drop shadows, and bold or italicize. Then again, some of Portable Scribus' extras, like a built-in barcode generator and educated color wheel, pleasantly surprised.
Is Portable Scribus a worthy download? Absolutely. Will it shake the foundations of professional media creation companies like Adobe? Probably not, unless mobile EXEs become of the utmost importance. It is, however, an alternative, and a gift to the user community that will benefit many users many times over.