eBay and Craigslist
Mind your eBay reserve: Tiptoe through starting prices when auctioning off an item to save money. If you're pricing something near eBay's thresholds of $1, $10, $25, $50, and $200, cut back by just a cent to save yourself a little cash, which can add up for high-volume sales.
If you list something at $199.99 for example, eBay charges $1. For a $200 starting price, eBay charges $2. For more information, check out the eBay Fees page.
Spell poorly: If you're buying hard-to-spell product, try searching for misspelling or common typos ("playstaion"), or enter * as a text wildcard such as "playst*". (eBay will find hits for one or more additional characters.)
You might stumble over auctions that others haven't found--which means less bidding competition. If selling, enter a few of those errors into your listing to snare misspelled searches (though probably not in the title--you don't want to disappear from the correctly-spelled search results).
Search Craigslist with RSS: Keeping an eye out for a free leather couch on Craigslist? Save yourself from searching every 10 minutes by setting up an RSS feed for the search results by entering your search within Craigslist and clicking the RSS button in the lower-right to add it to your RSS reader of choice.
Entertainment Extras: YouTube, Flickr, iTunes, and Xbox Live
Download YouTube : Watch online videos away from the Internet by downloading them to your PC. The simplest methods usually work best for noncommercial video, such as random YouTube home movies. Play clips back with VLC.
Watch high-quality YouTube: Bump up the resolution in supported clips. Just click the number and arrow in the play bar; it's likely 360p by default. Pick a higher number for the best quality.
Link straight to part of a YouTube clip: Instead of sending people to the beginning of a clip, you can create a link that plays from a spot in the middle that you choose. Just add #t=[number]m[number]s to the end of the URL, such as http://www.youtube.com/watch?v=Mf6IQgFxpIA#t=00m11s to begin 11 seconds in.
Initiate console game downloads: Microsoft's Xbox Live site might seem superfluous on a PC. However, you can queue up downloads and activate purchases through your computer, and when you log in back on the console, they'll transfer automatically.
Upload to Flickr through e-mail: Instead of manually uploading photos, you can send them through e-mail. This can work well from an old mobile phone to quickly send pictures from a PC. In the Emails & Notifications tab of the Account options, click Create an upload-to-flickr email address to configure the feature.
Search and link to iTunes content in a browser: The iTunes store lives in its own application, but you can access specific apps, songs, searches, or anything else with a URL. To search, try http://itunes.com/[search term] such as http://itunes.com/evernote. If your search comes up with a single result (as in this example), you'll jump to that page within iTunes. To copy a direct URL to an iTunes store page, right-click an item within iTunes, and choose Copy Link.
Feel free to share your own tips and tricks in the comments!