Easy File Scramble is a fast and free app to scramble and de-scramble files. This prevents the files from being opened in file viewers, zip extractors, etc. Easy File Scramble uses a two-way encoding algorithm, so there is no extra data stored about the files encoded and Easy File Scramble has no knowledge of whether a file is being encoded or decoded.
You can download Easy File Scramble on the Android Market here.
Update 4/2012 This is now open source and hosted on GitHub. Check it out at my GitHub page.