VLC media player is a great alternative, and replacement to Windows Media Player, or Quicktime. Not only does it play all video formats that quicktime, and WMP can play, but also plays nearly any video, or audio format i've been able to find. This is one piece of OSS that has proven to be software than either WMP or Quicktime.
Open Office is a great alternative to Microsoft office and other office suites that are available. It was designed to nearly replicate what the Microsoft company has done with Office, but the product is offered freely. Some Microsoft Office fans will notice changes and may be unhappy with the differences, but OpenOffice can handle most things that a student will utilize on a day-to-day basis. The only issue i have found is the inability to save in .doc, or .docx formatting which can upset professors when trying to turn in assignments as everyone would like to have the same file format.
Gimp is a great piece of OSS that is a decent alternative to shelling out major money to purchase a license for other photo-editing software. Before using Gimp, i was familiar with Adobe Photoshop. I utilized photoshop for editing photos for the most part. Gimp stepped in remarkably well after I lost my license with Adobe. Gimp is not nearly and powerful, and deep as Photoshop is, but for simple editing, and cropping of photos, it allows the user to work easily, and produces results similar to that of photoshop.