merge the code, write a "switchboard", so that the user gets to choose which functionality to run, ...
if you want a better explanation, you may want to be more specific.
are you talking about merging the functionalities of ten applications into one application,
or do you just want to make the functionalities of those ten applications accessible by one application? there's a big difference there.
hmm, write a modular system, turn each of the current functionalities into a module and plug them all into the system.
Something like OSGi comes to mind, maybe with an Eclipse based UI (or a web UI, depending on the nature of the intended system).