We just use a spreadsheet on Google Docs: we make a new tab every two months, start the top line with the current bank balance, and then just indicate deposits/withdrawals with a formula to display the new total next to it.

We tried doing some fancier stuff, but it didn't take. This way, we have a column for the date, one for the +/- amount, one for the new total, and one for a description of the transaction.