I've found that quite simply writing about a side project can be enough to help "finish" it. There's something almost cathartic about the process of writing up and publishing. It isn't about other people reading what you have written, and personally I don't mind if no-one else does actually read it. Rather, it is about gathering your thoughts on your objectives and how it has turned out, documenting it so it is left in a state where it would be a lot easier to pick up at a later date, and that sense of finality or closure (it means that, if nothing else, I at least got it complete enough to write about and also completed the writeup).