Using Others Work - To What Level Is It Okay? From September I will be starting my final year project at a UK university. 

I will be developing some software. However the software I wish to create has multiple "modules" to it. Approximately 16 modules for 16 different functions. 

A fair few of these modules have been created before by various people and the code is completely open source with no licensing restrictions. 

I will be writing a lot of the code myself, but I do not see the point in "re-inventing the wheel" by writing some code which will do the exact same thing as some software already out there.

Is it okay to re-use their code in my project? To what level is it okay to do this? 2/16 modules? 6/16? I do not want to go over the top into the plagiarism world. Obviously all work would be credited etc.
You absolutely must not use the work of others without attribution. If you credit everything you use in an adequate way, there is absolutely no limit on how much stuff from other people you can use. However, for your project to be successful, you will need to have a sufficient amount of your own contributions. To see what counts as sufficient, look at past projects and ask your supervisor.