Lala
Listen to any song in their extensive catalogue for free once, buy the 'web-only' version of the song for 10 cents, and buy an un-drm'd mp3 for usually less than 99 cents. The user interface is really nice, and loading the website in chrome is much faster than waiting and waiting for iTunes to load. You can even upload mp3s you already own to the website so you can listen to your library from anywhere. The 'follow friends' feature is pretty nifty too for discovering new music that your friends are listening to. There's only one downside - if you upload your music collection, you can't redownload them as mp3s, which means lala will not replace your existing music collection on your harddrive.
Hate your bank's website? I know I do. Bank of America has a terrible UI and you practically have to pass an exam to log in. I understand they're concerned about security, but answering 10 trivia questions about my account before logging in is ridiculous. Mint allows you to monitor your finances in an easy intuitive interface. Not only that, but you get some great meta-information about your account for free - how much you're spending on food per month, graphs of your spending from month to month, etc. They were really useful when they sent me an email saying, "Did you know Bank of America just charged you a 220 dollar service fee?" I did not know! Luckily, I sorted this out with the Bank and thanks to Mint I'll know if something like that happens again.
Found out about this one from last week's episode of Diggnation. Basically, you win prizes for free on the internet - and for the first time, it's not actually a scam! They are ad-supported so you just go to the website, get raffle tickets for prizes, and then check back when the raffle ends to claim your prize. Additionally, some profits from each raffle are given to charity, which means not only do you win free stuff, you feel good while doing it. I'm definitely going to be spending a lot of time on this site.
Any cool new web applications that I'm missing? Leave a comment and let me know.