In sophomore year of college, I was put in charge of accounting for my six-man apartment's communal finances. This could've been a complicated and time-consuming task, given that a) we had dozens of joint expenditures per month, b) different people paid for different expenditures, and c) sometimes, expenditures would only apply to a subset of the six of us.
Therefore, to make all our lives easier, I developed a system, dubbed “Project Apia”, that would track how much we owed each other. With Apia, and I could simply enter expenses (e.x. the power bill) and see how much money we were owed/owed others.
Apia had three main components: expenditures, debts, and summaries. In expenditures, anyone who paid for a communal item could enter it into the system, checking off only those housemates relevant to it. On the same page, that individual could also view and filter through a list of all the expenditures they paid for. At the other end, on the debts page, each user could view and filter through a listing of debts they owed to others.
The most convenient part of Apia, however, was its home/summary page. My housemates and I would reconcile debts at the end of each month, and this page automatically calculated and displayed each individual's net expenditures, along with who he was a net creditor or debtor to. I also added in a convenient "Give 'em a bailout" button for a simple, one-click bulk action to mark all relevant balances as reconciled.
Overall, Apia made communal/personal finance and expense auditing a lot easier for everyone in our apartment. It enabled all of us to pitch in by entering data, provided universal transparency, and saved me all the time I would've otherwise spent swearing at Excel.
For anyone who's wondering, I did have some plans to commercialize Apia, but unfortunately I waited too long before seriously considering them; now there are apps out there that do the same thing Apia did (although they weren't out when I first released it). Lesson learned: the tech world moves faster than you know it!