WOw, what an interesting question? What size of poster are you talking about? I am pretty certain ANY software will do, as long as you can set the DPI, and PRINT SIZE!
If this is promotional poster, then stay away from free software, as you get what you pay for, and IT TAKES MONEY TO MAKE MONEY! If you are expecting to draw people in with a poster made on free software, then you aren't expecting to succeed.
Look into an ADOBE product, and you will not be disappointed by how flexible and easy to use they are!