Too Many Services

Owing to bugs in the service system of Mac OS X up to version 10.2.8, the Services menu might disappear in Carbon applications (like the Finder) if the number of service items exceeds a certain number (usually around 60-70 items). If that is the case, just remove all unnecessary (unused) applications providing services, or use an application like Service Scrubber to disable some unnecessary Services menu entries.

Also, the APE (Unsanity's Application Enhancer) system extension may block services from appearing correctly in the DEVONthink Services sub-menu.