EducationalNote that this list is mainly targetted towards windows users. That's not because I think windows is better, but it's because I know linux users already know this type of information ;).
Replace the bulky, unsecure bloatware called microsoft office with this freeware clone. Yes, if your computer crashes it makes save checkpoints every few seconds to recover data.
http://www.openoffice.org/Do you like to open rar, zip, and tar files without being prompted to register? Download 7-zip for a fast light weight compressor/decompressor of files.
http://www.7-zip.org/Do you value time, sanity and a fast computer? Download foxit reader so you can download pdf files and read them instantly. Adobe acrobat frequently crashes and makes my fast computer sound like its grinding away while lagging horribly trying to process the monster program.
http://www.foxitsoftware.com/pdf/rd_intro.phpReplace your graphing calculator with your computer.
http://gcalc.sourceforge.net/Similar to winkey. Allows you to quickly and efficiently configure quick keyboard shortcuts to a variety of tasks.
http://hotkeybind.sourceforge.net/download.htmlUsed with plugins this is a fast light weight tool to watch all kinds of video formats. In recent years many hackers have targetted windows media player because of many glitches in the way it processes streaming media. MPC uses much less memory making it way faster for students who like multitasking.
http://sourceforge.net/projects/guliverkli/Are you a hardcore instant messenger? With gaim. all at once, you can connect to AIM and ICQ (Oscar protocol), MSN Messenger, Yahoo!, IRC, Jabber, Gadu-Gadu, SILC, Novell GroupWise Messenger, Lotus Sametime, and Zephyr networks. Gaim supports plugins!
http://gaim.sourceforge.net/about.phpThe p2p trio can help get archived tutorials and instructional materials. P2P software CAN be educational. I found it very useful for learning french and finding sheet music (searching archives on emule). Be sure to check copyrigths before you download something =P. This combination of freeware is probably the most efficient.
http://www.emule-project.net/home/perl/general.cgi?l=1&ga=1&rm=downloadThis will help suppliment #8. Believe it or not there are now large and commercially competitive freeware antiviruses. For individual files you want to scan for viruses try Jotti Malware Scan and Virus Total.
http://www.clamwin.com/Little known powerful file that allows you to configure the windows key in windows. You can go to your favorite sites, open common files, and play your favorite music at the combination of two keys.
http://www.pcworld.com/downloads/file/fid,5506-order,1-page,1-c,alldownloads/description.htmlTodo lists, calendars and such.
http://sourceforge.net/search/index.php?words=flash+cards&sort=score&sortdir=desc&offset=0&type_of_search=soft&form_cat=18Many students learn material with flash cards. There's been thousands of flash card like programs made to help you memorize content quickly and easily. In fact, I even made an online quiz maker myself in php, though it's not the most straight forward to learn >_<.
http://pauker.sourceforge.net/pauker.php?target=download&lang=en