"Look at this over my shoulder" might be good enough if you want to share a document with someone in the cube across the hall, but when you need to review a document with someone in a another zip code, you need something like ShowDocument, a free Web meeting service. Check out the video after the jump.
As this gratuitously long video shows, ShowDocument requires no registration or time-consuming setup; just choose a file to show, and then send participants a 5-digit code to enter your meeting. The site supports all the usual file formats -- Word, Excel, PowerPoint (though not the latest Office 2007 variations), PDF, and images, as well as synchronized Web browsing, Google Maps, YouTube, and a plain white board. There's text chat and a number of tools for interacting with your document, such as pens, markers, an eraser, and the ability to save the finished product -- with markup -- as a PDF.
I like ShowDocument, but the free Web collaboration business is more crowded than the ladder closet at the zoo's giraffe exhibit. Don't forget that Twiddla lets you create math formulas, and DimDim includes full-up desktop sharing.