Gwen is a former servant. She appeared as one through from the start of Series 1 to episode 9 of Series 4. She was a servant briefly in Episode 13 of Series 4, until Arthur crowned her Queen of Camelot.
Nimueh once disguised herself as the servant "Cara".
Many of the female servants, in fact the majority, wore turbans on there head. Guinevere, however, did not. Nimueh used the turban to disguise herself.
In Episode 1 (The Dragon's Call) Merlin defended Arthur's current servant, whose name is Morris, but he appeared to have slightly long, messy brown hair.
Merlin and Guinevere (when she was a servant and of course now) are trusted members of the court. They are probably the only servants who are, considering how close they are to the highest powers in Camelot.
Interestingly, the servant of lords, such as the former lord Agravaine, are not seen.
It is implied that knights have servants of their own in their households. Guinevere's mother is said to have been a servant in Sir Leon's household. (The Coming of Arthur) The knights' servants may have only been limited to their households, or they may ask for them to never be around during meetings with the prince or court sessions.