I'm aware of many sites that you can use for cross-browser testing and I was wondering if anyone knows any free sites that you can use to test sites on an iPhone?
You can use Safari (free download on both Mac and Windows).
iBBDemo is visually helpful for showing iPhone and iPad page rendering to business users. http://labs.blackbaud.com/NetCommunity/Page.aspx?pid=249 It is not as accurate at using the SDK, of course, but it is free and simple, and works for us Windows users.
Also, if you use Adobe Dreamweaver, check out their integrated "Device Central". It has an very large library of mobile device specifications that you can leverage to see how your page will appear on the Mobile Web.
Head to Apple developer, create a free account and download the SDK (Mac intel only though). Install it and launch the iOS simulator. You'll be able to test it on an iPad and an iPhone... all from your computer. Thing is: it might be a little overkill: the SDK is huge for what you're trying to accomplish.
You can also try a Webkit browser (such as Safari or Chrome) by resizing the window to 329*480 but you won't get the same rendering as the mobile browser. As a general rule, stick to standards and you're site should render fine on recent mobile devices.
The browser on the iPhone/iPad/iPod all use Safari, the same Safari that is used on the Mac and the Windows machines.
All you would need to do is change the window size of Safari to be the same as that used by an iPhone/iPad/iPod.
This website has a tutorial on how to do it. It's quite clever and easy to do.
If its short term you can use a free trial from http://keynotedeviceanywhere.com/index.aspx