My wife was interested in trying stumble upon so she joined and then sent me an invitation. When I joined it looked at my contact list and had pre-checked a couple of email ID's. I unchecked all but my wife's and one other person whom I thought would be receptive to the whole thing. I clicked continue. The next day all these people I hardly knew and had no intention of inviting to look at my choice of web sites were emailing me asking me why I gave this site their email address. When I emailed SU to ask them to make it right they acted as if I did it voluntarily so that it was not up to them to fix it. I cancelled my account and hope to never hear of them again. What a bad experience. We all hate spam unless your in the spam business. They must be because they did not seem to be concerned one bit about what they did. This product needs to care more about spamming and less about making money.

Yet another web 2.0 service, but this one really introduces a new way of browsing the information superhighway. Stumbleupon is a good toolbar.

Updated review - September 9th 2005. Updated to prove that this is my review... I posted it here first and then in the Mozilla add-on assessment page. I am now being accused of plagiarising my own work - which is kind of difficult, no?
Also available at the following sources (all of which are MY accounts):
http://www.ciao.co.uk/stumbleupon_com__Review_5528003
http://www.reviewcentre.com/review165912.html
Stumble Upon works on all browsers except Safari, IE for Apple Mac, and Opera. Published by Mozilla, it is a free add-on which lets you browse, review, and share a huge range of sites, while meeting new people.
Choice and variety are the best aspects of StumbleUpon (www.stumbleupon.com). The underlying idea is that you select topics that interest you. You then use the stumble button to visit related random websites, which are reviewed by other stumblers. You can either agree or disagree with their ratings and post your own comments; or you can simply move on to the next site/topic. There are hundreds of special interest groups here, where you can 'meet' like minded people.
StumbleUpon is community based without being claustrophobic, as you can interact with others as much or as little as you choose to. Your page can be used as a normal blog: you are not obliged to rate any websites if you don't want to, or if you aren't interested in them. It's worth remembering though, that the growth of StumbleUpon's database relies on input from its users. Submitting interesting, high-quality sites also improves your rank as a top stumbler, bringing your recommended sites to the forefront of the database.
Although the site is free for all to use, you can gain extra access by paying £11, or $20 per year. The benefits of upgrading include the ability to turn off advertisements, unlimited stumbling within favorites of your friends, and the option to create new interest groups. Unless you want an exceptionally high level of involvement within the site's community development, upgrading isn't particularly useful: you can find links and contacts almost as easily by registering for free.
I highly recommend this educational, innovative toolbar: It is the most informative browsing extension I have seen so far. It is useful for people of all ages, and is a lot of fun! The only reason I have NOT given StumbleUpon 5 stars, is because it is currently having appalling server issues. It was running ridiculously slow yesterday, and now has stopped working completely. This is rare, though. Although the program has occasional bugs, these are usually solved quickly. In terms of Spyware: I have not encountered any Spyware myself within StumbleUpon, nor have I been told of any such problems by other regular members.

StumbleUpon (www.stumbleupon.com): Choice and variety are the best aspects of the underlying idea is that you select topics that interest you. You then use the stumble button to visit related random websites, which are reviewed by other stumblers. You can either agree or disagree with their ratings and post your own comments; or you can simply move on to the next site/topic.

Although the site is free for all to use, you can gain extra access by paying a small amount ($10 per year). It is community based without being claustrophobic, as you can interact with others as much or as little as you wish. There are hundreds of special interest groups within the database, where you can 'meet' likeminded people. You are not obliged to rate any websites if you don't want to or aren't interested in them. It's worth remembering, though, that the growth of StumbleUpon's database relies on input from its users.

I highly recommend this educational, innovative toolbar: It is the most informative browsing extension I have seen so far. It is useful for people of all ages and is a lot of fun! The only reason I have NOT given StumbleUpon 5 stars is because it is currently having appalling server issues. It was running ridiculously slow yesterday and now has stopped working completely. Let's hope this problem is sorted ASAP!

StumbleUpon is the most addictive thing I have found on the web in all my time of using it - even more additive than trying to hit a falling penguin with a club!

Once you've downloaded the toolbar from their site you're ready to embark on leaving little footprints 'underneath' each and every website out there.

It's a very simple idea, you either give a thumbs up or a thumbs down to each site you visit, with the option of leaving a little message to other stumbler's that erm... stumble upon the site.

Ok, other people can see where you've been, come in from, but it doesn't overly matter as you then find other sites of a particular interest to you that you've never seen before.

The more you contribute by voting, adding comments to web pages, submitting threads to stumble forums the more you're rewarded by an audience counter,

The counter goes like this.

Level 1 = 1-24
Level 2 = 25-50
Level 3 = 50-99
Level 4 = 100 - 199
Level 5 = 200+

How they determine the number of points you get for each submission is anyone's guess, but it certainly makes you rate each site you come to in the vain hope that it will take you up to that next level!

It's is completely free to use although they do give this rather innocent line of

"Instead of harassing our users, we prefer donations from people who enjoy our service. People usually make a donation of $5.00, $10.00, or $25.00. This pays for our bandwidth, helps support our developers, and keeps StumbleUpon ad-free!"

So download and enjoy

At time of writing the review I was a level 3! :-)