There are times when the third umpire is unable to come to a firm conclusion about a decision, even after the use of multiple replays, hotspot, etc. What should happen in this case?
There are two different scenarios here:
The on field umpires refer a decision to the third umpire because they are unsure of the correct decision. In this case, the benefit of the doubt goes to the batsman, which generally means "not out".
The third umpire is brought into play after the use of a DRS review by one of the teams. In this case, the third umpire must have conclusive evidence to overrule the on field umpires, so if the third umpire is unsure, the decision made on the field stands.
References: ICC Playing Conditions, Appendix 1 (Decision Review System), Sections 2 (Umpire Review) and 3 (Player Review).
Update: DRS has specific rules when to go with 'Umpire's Call' but for non-DRS referral the on-field umpires are nowadays giving a 'soft signal' while referring the decision to the third umpire: their own opinion whether out or not out (with the possible exemption of line decisions like run out, no-ball, stumped, and boundary line decisions) -- if the third umpire is unable to conclusively overturn the soft signal, then the soft signal becomes the final decision.