What part-time jobs are suitable for PhD students? I know PhD is unsparing in terms of available time, but at times there may be situations that demand some extra income, for example, family situations or some medical emergency. 

In such cases, are there any specific jobs PhD students could take up? This could be at some place within a university (a library, say) or somewhere in the city (assume it is a large one like London or NY). The desiderata are minimal time consumption and a sum of useful money which could be used to manage the unexpected expense.
Two obvious answers are:


Marking (grading). Ask your supervisor if they have coursework that needs to be marked (for a price).
Tutoring. Put a note up in your departmental office offering to tutor undergraduates. If you're doing a more mathematical degree, you could tutor undergraduates from other disciplines. For example, helping out with some statistics coursework.