There's a Google Doc template that I found to make a loan amortization table. It shows you the progress of your loan on a monthly interval; How much interest and principle you pay each payment and when the loan will end.
I took this template and I made my own template of something that would work for my needs. I wanted something that would also allow me to add additional amounts at different time periods to see how that would affect my loan. This way I could see how much less interest I would pay in the long run. I made two extra columns, one for a regular amount (so if you payed $30 more than your payment for a year) and a second column for one time larger amounts (maybe from a tax return).
I also simplified the amounts at the top that you as the user plug in to make it a little more straight forward.
Finally I've added a column at the top that shows you the amount of original interest that you would have paid by making all of the regular payments, the new amount based on the extra you've paid on your loan, and the difference in the two. This way you can feel good about putting extra money against your loan!
Hope you all enjoy this spreadsheet as much as I did improving and using it!