To export your database, first adjust your view settings (view bar, focus, sidebar selection, and so on) to show the data you want to export. When the window contains the data you want, choose Export from the File menu, then choose a format and a location. The available formats are: OmniFocus Document — This is an ordinary OmniFocus document, just like the one that you use as your database. If you open such a file in OmniFocus, it appears in its own window and you can work with it normally, but application-wide settings such as preferences and perspectives don't come along with it.
Plain Text (TaskPaper) — This is a plain-text representation of your data, and it happens to be compatible with the light to-do application TaskPaper from Hog Bay Software. Of course, you don't need TaskPaper to use your plain text export; you can just give it a .txt extension and open it in the text editor of your choice.
Simple HTML — This is a single-file HTML representation of your data; the stylesheet and even the icons are embedded in the HTML. If you are proficient with CSS, you should be able to restyle the result however you like.
Comma Separated Values (CSV) and (CSV UTF-16) — CSV is a kind of lingua franca for applications old and new on all plaftorms, as it's just all of your data in a plain text file with its columns separated by commas. Once you have your data in CSV format, it's easy to run scripts on it, convert it to some other format, or open it in applications that understand it (like OmniPlan!). If you're having trouble persuading other applications to read the non-ASCII characters in your CSV file, such as accented letters or non-Roman characters, try exporting with the UTF-16 CSV option, and importing that into the other application. This makes it easier for some applications to detect the correct encoding and interpret your characters properly.