1. System Performance
2. Online Services

1. System Performance

Turn on Clear Type
If you are running XP, you can use this site (you will need to access it in Internet Explorer) to clear up the blurry fonts on your display.  Clear Type is set to be on by default in Vista; it is not available for pre-XP systems.

http://www.crucial.com
This site will scan your computer and tell you exactly what RAM you need--for free. You can either take that info and go elsewhere, or simply buy it (at a relatively cheap price) from them.  Crucial used to supply RAM for Dell. I have used them in the past for the only RAM upgrade I've ever done and I've had no problems.

EICAR Anti-malware Test File
Sometimes you aren't too sure that your antivirus program is working. You can go here and download a (supposedly) harmless sample virus to make sure that your av program picks it up.  

2. Online Services
http://www.imageshack.us
Imageshack will host your pictures for free.  If you register, you can have them hosted permanently (whatever that is supposed to mean in internet terms); but even if you don't, they will host it, as long as someone views the image at least once a year.

Keyboard Practice
Title says it all, apart from the fact that it will give you a words-per-minute score, and pretty much, let you practice at your own pace.  Another site you might like is mytypespeed.com.

http://www.theultimatesteal.com
This is a Microsoft website, that offers college students taking at least half a unit, with an .edu email address, the most complete version of Office 2007, for $60, or $75 if you want the CDs.  Do keep in mind that this advertised price has been announced as good only through April 2008; Microsoft might extend this deadline, but they might not. In early August 2008, the site said that the offer would begin again later that month (August 2008), so stay tuned.

Virus Bulletin
This website tests similar versions of antivirus programs and reports how well they did keeping a computer free from viruses.  They archive their reports by company and platform (i.e., version of operating system).  If you decided that you want to compare how a variety of antivirus programs would do, protecting a computer against the same threats, this is the place to go. They do require a (free) registration, but it's worth the hassle.

FileExt
FileExt gives you information on pretty much any file that is out there.  If you ever get a file and aren't sure what program will open it, or what type of file it is, this is the place to go.

Web Archive
I'm really not sure how they manage to do this, but this site saves images of websites, so you can access them even if the original site goes down (as in permanently).  This is particularly helpful when you are searching for an article, only to find that the site that hosted it is no longer running.  Now the Web Archive doesn't get every single website out there, but it does get quite a few.

Amazon.com
Now I know we've all heard of this website, but there are two reasons why it can be helpful (and why it definitely has been helpful for me).  First of all, amazon lets customers of particular products (who have registered with amazon) write reviews of anything that it sells.  Looking at customer reviews (before you buy, even if you don't have an amazon account) can be quite helpful if you ever decide to buy something, but don't have the opportunity to try it out first; or if you are simply comparison shopping.  A second way that amazon.com can be helpful (and one that many, but not all people know about) is that in addition to selling new books, they also sell used books (in case it hasn't dawned on you yet, they have yet to arrest someone for buying their college textbooks used online)--usually, at a fraction of the price of the same book new.  When shopping for used books, keep in mind a few things. One, the booksellers will rate their books with a variety of descriptions: "Acceptable." "Good," Very Good," "Like New," or even "New." They also might provide a desciption of each particular book. As a rule of the thumb, you will probably be better off paying an extra couple of dollars for a book, if it will be in "Like New" condition, as opposed to "Acceptable." Two, pay attention to the feedback of the used bookseller.  If I am going to buy a used book, I'll probably pay an extra 50 cents for it if its from a seller with several hundred feedback, with a 99% positive feedback rating, rather than someone who just started selling, and might not know too much about shipping the book, or how to accurately describe it--or rather than from someone who has an 80% positive feedback rating.  Of course, your mileage may vary.

KeepVid
This site will try to download videos for you, even if there is no option for you to do this at a particular site (such as Youtube).  They don't guarantee a download, but the few times I've used this (free) service they have usually come through.

YouSendIt
YouSendIt requires a registration (free), but will allow you to "send" files up to 100 megs in size (most email accounts limit you to 10 megs, if not less).  What happens is that you upload the file to YouSendIt, then you email a link to someone, and then they can download it.  Now the downloads and uploads are a bit slower than most that you will do, but this still can be very handy.

OK call me a smart-aleck but I really mean this.  You'd be surprised at the amount of information you can get simply by searching for the asnwer on a good search engine.  

OldVersion.com  This site keeps old versions of software programs on its servers for you to download. This is helpful when you get an older computer, or uninstall a previous working version of a particular program, only to find that the newer version just doesn't cut it for you.