1) They are interested in coding. This was done on their own time, away from work. This means they actually like to code.
2) A lot of work is done to learn a new language/technology. This often shows desire to learn and the ability to at least hack together something that is new to them. This is important because we will be asking them to come up to speed quickly in our code base.
3) You can look at the code and get a feel for how they code. We are a small team and each developer will have the ability to have a big impact (for good or bad). Getting an idea of how they think, how they solve problems is a huge plus.
I don't think you need to have a lot of code available. Pick your favorite project, or 'best' project and make it available. I see very little downside.