The nearest approach to the complete solution is the sciposter class, which provides the means to produce really rather good posters according to the author's required style. A complete worked example is provided with the distribution
Otherwise, there is a range of tools, most of which are based on the a0poster class, which sets up an appropriately-sized piece of paper, sets font sizes appropriately, and leaves you to your own devices.
Having used a0poster, you can of course slog it out, and write all your poster as an unadorned LaTeX document (presumably in multiple columns, using the multicol package), but it's not really necessary: the (straightforward) textpos package provides a simple way of positioning chunks of text, or tables or figures, on the poster page.
More sophisticated is the flowfram package, whose basic aim in life is flowing text from one box on the page to the next. One of the package's design aims seems to have been the production of posters, and a worked example is provided. The author of flowfram has an experimental tool called FlowframTk (formerly called JpgfDraw), which allows you to construct the outline of frames for use with flowfram.
The beamerposter package is added to a beamer document to enable the user to work as if in a a0poster class. Thus beamer's neat provisions for layout may be used when creating the poster. Documentation of beamerposter is sparse, but an example file allows the user to get a grip on what's available.
Despite the relative shortage of tools, there are a fair few web pages that explain the process (mostly in terms of the a0poster route):