Can I make use of the problems elsewhere?
Yes! It is not only granted but it is encouraged for material to be shared and used freely for non-profit making purposes. But please read on...
All of the problems have been gathered from a variety of sources: some are based on interesting sub-results of larger mathematical theorems, others are variations of classic problems, the rest are believed to be original. The primary mission of Project Euler is to present high quality problems for the purpose of both encouraging learning and to challenge in an enjoyable context.
Are there any restrictions?
Considerable effort and thought has been put into the design of each problem, and in that sense all material is new and original. For this reason problems must not be used for commercial purposes.
Do I need to show attribution?
This issue is more a matter of courtesy. You are entirely at liberty to use the material as it is or adapt it. If you choose to use the content as it appears on the website you could say something like, "The following problem is taken from Project Euler." If you modify the problem then you could say something like, "The following problem was inspired by Problem xxx at Project Euler."
Do you have a way of linking directly to a particular problem?
You can link to Problem xxx with the following URI:
Can I get more detailed information on copyright?
The content on the site is licenced under a Creative Commons Licence:
Attribution-NonCommercial-ShareAlike 2.0 UK: England & Wales.
You can either read the Creative Commons Deed (Summary) or the Legal Code (Full licence).