I have been using Last.fm for a long time now, but I have always avoided StumbleUpon because I have been lazy and presumed that I wouldn't want it.
Today I installed the Firefox extension to see what it was like.
To put it mildly, I was impressed. After telling it what I was into, it was able to give me websites that I thought were really interesting and useful, though there are some which I disagree with subtly - too subtly for it's preferences engine to understand.

Open Source Software Alternatives

While I disagree with this site's "Open Source" focus, it seem a really useful resource for those making the jump to Free Software. For me, it shows what people use when they don't use Free Software, which amusingly I find that I honestly don't know.

Open Source Software for Windows

While this site seems to be somewhat chaotic with the author not really understanding "Open Source" (or Free Software"), it does have some lists of software for Windows. However lists like this tend to put off people and it doesn't really seem to be very helpful in my opinion.

Why Linux is Better

Despite being generally in favour of it's message, I find myself at odds with it, not least because it uses the word "Linux" instead of GNU/Linux but because it seem a bit outdated and seems a bit like it is talking down to the user. I prefer websites like www.getgnulinux.org

CB.VU

This is bizarre and cool. It is a Unix-like shell implemented in what seems to be JavaScript, perhaps AJAX. It seems to fit in a slot near www.sovix.org and www.eyeos.org. I may email them and ask them if I can have the Source under a nice licence - according to http://cb.vu/#info there is nothing about releasing it under anything.

Learn Shell

This website looks quite useful for guiding beginners to OS's which use BASH a fair bit. I know basics, but I'm thinking of taking a look at this site.

andLinux

It's name obviously irks me, though it is testimony to the power of Free Software to allow people to create mashups like this. What I disliked about this project was what I found when i searched for the licencing.

This software is covered under the GPL. Use this software at your own risk! No fault will be taken by andLinux or anyone involved with andLinux.
You may not sell andLinux, but are welcome to distribute it freely. This is not a GPL violation as the andLinux name is owned by David Solomon.
You may not use the andLinux name without written permission from David Solomon.

The bit I have made bold is the bit I object to. As far as I am aware the GPL allows you to distribute the software for money so long as there is an offer of the source code.
While the part regarding the name is possibly true, I suspect this varies greatly from country to country and while it may or may not be at odds to the GPL legally, it certainly is morally. Not allowing people to charge to distribute software seems directly at odds with the GPL from my point of view. It would be interesting to get someone from the SFLC to take a look at it.

What to do when Xserver freezes

This looks quite useful to be honest. It tells you what to do when your GNU/Linux system doesn't respond to Ctrl + Alt + Backspace.

BitNami

To be honest, I don't know what to make of this. it's tag line is installing Drupal, Joomla etc in under 10 mins. With sudo apt-get install drupal5 -y, I haven't had difficulty installing Drupal in under 10 mins.
However it does offer all sorts of things in these "containers" which allegedly make it very easy to install loads of stuff on various platforms. I don't see the point. It seems to me that this is likely to cause more problems than it will fix, especially on GNU/Linux.

Subnet Mask Info

To be fair, if you need to do most of the stuff this set of JavaScript Calculators does for you, you can probably already do it in your head. If you don't know how to do this stuff in your head, but need to use it, learning it might be a good idea.

iBypass

I dislike censorship. That includes content filtering. I have probably circumvented, on occasion, most of the content filtering systems I have encountered to do things like check my mail. I was therefore slightly amused when StumbleUpon recommended http://www.ibypass.com/ to me.

Bypass Content Filtering with SSH

This article talks about how to bypass content filtering with something as basic as SSH. If one can run programs on the host computer and change browser connection settings, then this was is probably preferable, however it does require a certain amount of technical expertise.

New Revolution

New revolution seems to be some crazy activist site. I can't say I totally agree with it's grassroots political attitude however it presents a few interesting ideas.

Browser Shots

What more can I say. If you are a web designer, this service, licenced under Free Software licences, operated by volunteers around the globe, is AWESOME. It take screen shots of your web browser with a frighting array of webrowsers on a mass of operating systems and then lets you download them. Here are some screenshots of blog.tdobson.net. *When* I have money, I'll support these guys for the awesome job they are doing.

How not to make UI mistakes

The geekup crowds would like this, though I find it unfunny really. It does raise some points. Don't mess up your UI - use your common sense. See what you think.

25 books for Web Designers

This blog post seems to cover the depth and breadth of web design, however not having read any of the books I can't say whether they are useful. It is interesting to note that the blog author is using the links as income stream as he gets money for referrals from Amazon. Several of my Friends does this, perhaps I should start. I don't fancy adverts on my blog to say the least. At least, not until it is wildly popular.

Pocket Guide to CSS

This CSS Cheatsheet seems to have pretty much all the important elements of CSS on one page.

CSS Page layouts

For Newbie Web designers, this is really useful. It gives you a small piece of CSS lets that do all the layout to avoid people using tables to create horribly aligned websites. This is the first site I have seen which is actively trying to combat people writing newbie website sites in a positive way. Kudos to the web(master|mistress).

MiniAjax

WOW. This site rocks. It lists a *large* number of AJAX projects and programs which do neat things in a simple way. My favorite of those I looked at was GreyBox, "A pop-up window that doesn't suck." I *really* like this site.

Web Developer's Field Guide

Whew! Thats a lot of links. It basically links anything a web developer could possibly want to look at when building something which might go near the Web. This page leaves no stone unturned as it tries to provide links to *everything* one could want. Magically, it probably does.