Reveal RootKit
Reveal Rootkit detects processes hidden by rootkits on POSIX systems.
Reveal Rootkit detects processes hidden by rootkits. It is intended to run out of cron or similar services on a regular base and avoids verbose output as long as nothing was found.
It's fast and shouldn't produce false positives.
Reveal RootKit is tested mainly on Linux but should work on other POSIX systems with a /proc filesystem, too.