24. Greasemonkey

Greasemonkey is a browser add-on (extension) for the Firefox browser (no equivalent in Internet Explorer, sorry) that allows little snippets of JavaScript code to manipulate the web pages visited by the browser. After you install the extension, you can then use it to install and manage scripts that do everything from change ads on pages from the original ad graphic to random Flickr pics to adding information from one site (say, your local public library's) to another site (say, Amazon.com). This is all done via the JavaScripting language. You aren't a JavaScript expert? No problem, people have been writing scripts - and sharing them - for years. Just about any site that gets more than a couple of visitors a day will have some scripts that can adjust its user interface, functionality or both. You can find an exhaustive list of scripts, arranged by website, on the Greasemonkey User Scripts wiki or you can find a number of "best of" lists that link to the most popular, useful or fun scripts that author can find, some specific to particular sites like del.icio.us or Gmail.
I use Greasemonkey to add a "twit this" link to my Google Reader web page - giving me a quick way to share what I'm reading with my Twitter buddies. I also use the Better Gmail script to improve the way the already very cool Gmail service works for me! Spend some time browsing around the various script sites, seeing what is available and what might improve your browsing experience (pay attention to all the social sites that have scripts - FriendFeed, Twitter, del.icio.us, blogger, Facebook, etc. - some of them are handy in helping with information overload).

12. Google Maps

Google Maps deserves it's own lesson - it's a Google application, true, but it has spawned more mashups (2 sources of data "mashed up" into one useful application) than any other application out there. Google recently released a personalized version of the Maps application that lets you very easily create a new map with your own data in it as well. I created this one:


View Larger Map

in about 10 minutes on the last night of my recent vacation. Other people have created maps that take a source of data (gas prices, for example) and combined that with Google's Maps to create Gas Buddy, as one example of a mashup application. There is a directory at Google with all the popular maps available for the US (or whatever geographical area you have in the screen).
There are at least two blogs that track what new mashups are being created with Google Maps as well.
Creating your own mashup, as I did above, is very easy. Go to maps.google.com and click on the My Maps tab. If you are already logged in, you will see a "create new maps" link. Click on it, title the map and add a description in the sidebar on the left side of the screen. Once that is done, find a place of interest on the map on the right, either by clicking and dragging to the location or doing a location search, and click on the 2nd icon at the top of the right frame (the blue "placemark" icon) and click on your location. You can then add a name and description to your place. Do that for all of your locations, and your map is done! I added the images into my map descriptions by copying the image address from the Flickr photo page, using the "rich text" editing in the description of the placemark and clicking on the image icon to paste the image address in and make it show up on the description of the map.