Please upgrade to stable build 5077. You can do this by using the update.sh utility on *nix, or freenet-webinstall.exe on Windows, or by downloading the jar from here. This build merges an enormous quantity of work from the unstable branch, including a fix for a very serious bug affecting inserts, some optimizations, refactoring, many more bug fixes, a substantial reduction in memory usage with the fixing of many memory leaks, and a major change to how freenet nodes work together ("bidirectional routing"). Like previous builds, there will probably be bugs in it, and we would as always welcome bug reports at support@freenetproject.org. We will not see the full effects of the changes until most of the stable branch network have upgraded, so please upgrade ASAP. If you don't have a Freenet node installed, now is a great time to download one. We are trying to work towards getting a 0.6 prerelease out, but recent network tests suggest we may need to deal with one more major network level issue first (but 5077 should nonetheless be a significant improvement). We will need to do more debugging, of course, and we may or may not implement automatic IP address detection for firewalled nodes before 0.6.
http://www.freenetproject.org/index.php?page=download