GNU ext2resize is a package which allows resizing ext2 and ext3 filesystems (both shrinking and growing). The ext2resize tool is for resizing unmounted filesystems, and ext2online is for growing a mounted filesystem (it needs a kernel patch to work, however).
Please note that like any program that messes with your hard disk, this program has the possibility to totally destroy your disk, or even turn it into a sperm whale. There is NO WARRANTY for this program! See the GPL for the rest of the story.
Having said that, there have not been any reports of data-corrupting errors in either the offline or online resizing tools (at least not as far as I can remember).
Links:
Andreas Dilger <adilger AT users DOT sourceforge DOT net> Lennert Buytenhek <buytenh AT gnu DOT org>