Graduate admissions is based mainly on the question: Does this candidate's preparation suggest that he/she will be able and likely to succeed in our program? It is not awarded as a token of gratitude for service to the institution.
This project of yours could have a positive impact on a graduate application, to the extent the project itself is evidence of your readiness for graduate work (programming skills, initiative, etc). But from this point of view, it's not really relevant that you created it for your alma mater, or that your alma mater uses it, just that you created it at all.
(The fact that your alma mater actually uses it could have a small effect on your application at that same institution, if some of the people on the admissions committee are personally familiar with the product. They would have a better understanding of what the project says about you as a potential student. Of course this could cut both ways, depending on whether they think the product is good or bad.)
I would guess that this sort of thing would be a relatively minor factor in considering an application, and would not probably not save an application that was otherwise weak. But of course it will depend on your particular institution, their criteria, and the overall strength of their applicant pool. You could get a better sense by having a talk with one of the professors in your department who is involved in graduate admissions.