I don't use any online software however I can give you the low-down on what I use for my posters:
For general design and layout I use Microsoft Powerpoint (you can also download and use OpenOffice Impress for free). Its easy to use, widely available, familiar to the majority of printing services and templates  are available online. If you don't want to use a template just manually input your poster dimensions (File > Page Setup) and it'll print out at that size.
As for making technical drawings, it depends on what kind of information you are trying to communicate and how good you are with graphic design software. I use Excel for graphing as well as some built-in features of statistical packages. For drawings, I generally use a pen and paper, scanned and edited in Adobe Photoshop. Sometimes I even use Microsoft Paint. No lie, it may seem rudimentary but this method works for me and has produced professional, conference-quality work.
 Templates: www.posterpresentations.com/html/free_poster_templates.html
 General poster advice: www.swarthmore.edu/NatSci/cpurrin1/posteradvice.htm