Since I've had my iPhone, I've found a few things that I wish I could do with it. I wished that I had a fuel-economy tracker app for my car, an expense tracker for work-expenses and others. I knew I could've set-up a custom web-page backed with a database, but it just seemed that there should be an easier way.
It seems Google thought so too, as they've just released a powerful new Forms extension to their Google Docs app, Spreadsheets. Using Forms, you can essentially create a survey or email-based input form for a spreadsheet you've created. This has great implications for those needing to create ad-hoc surveys for your work-mates, friends, family and more. However, there's nothing stopping you from inviting only yourself to the form.
To get started, go to Google Docs
(sign-up if necessary), create a new spreadsheet. If you want, you can add a few columns with a header row to make things a little easier in the next step. Then, go to Share and invite yourself with the 'to fill out a form' option. You'll then be able to configure your form. Here you can add or delete fields, re-order them, and set each field to 'text,' 'paragraph-text,' 'multiple-choice,' 'checkboxes,' or 'choose from a list' (with the choices, where applicable). You can also set defaults in some cases, and add simple instructions.
You'll receive an email with the form embedded, but there's also a link to a web page version of the same form. On your iPhone, click this link from your email (or import this link via Safari). Once it is open in Safari, scale it to fit better on the screen, then press the + at the bottom of the screen and add the page to your home screen.
Now, whenever I fill my tank or make a business expense, I simply press the button on my iPhone, then fill in the form, and press Submit. All results appear in my spreadsheet. It appends a new row for each submission. I've also set up the spreadsheet to automatically take averages, or compute other figures from the submitted data. You just need to put these work cells at the top of your sheet, and refer to your data with deep-reaching cell-ranges like $C$4:$C$999, for example, so that the range will 'see' new data as it comes in.