www.mint.com
This is an amazing tool! Just load your bank accounts, your credit cards, student loans, investment accounts, etc, and it helps you keep track of everything. Sends you notifications when payments are due, helps develop a budget and protects against low balances and suspicious charges.
I just started using it today and I think the only drawback I've found is that it works better if you use credit or debit cards to make all your purchases so it can track them.