I discover new apps/sites everyday, but only a few are worth mentioning. Below is a list I've recently compiled consisting of web apps, iPhone apps, and desktop apps that I find useful. I will continue to add apps as I discover them.

Web Apps 

Notepad.cc

I discovered Notepad.cc on Hacker News a few days ago. This is a notepad app finally done right. It also allows you to password protect your notepad and gives you the option of using a vanity url (mine is http://notepad.cc/p). It's definitely a useful tool.

 http://notepad.cc

If you find yourself at a computer without Photoshop or just want to quickly edit a photo without having to open Photoshop check out Pixlr.

http://pixlr.com/editor/

Users Like Me

I'm usually not one to push things I've developed onto users, but if you use Gowalla you should check out Users Like Me. It will display users who check-in at similar spots you do.

http://userslike.me

iPhone Apps

Sleep Cycle

Sleep Cycle studies your sleep using the accelerometer built into your iPhone. Open the app, set the alarm, it will wake you up at your lightest sleep cycle.

http://www.lexwarelabs.com/sleepcycle/

http://itunes.apple.com/app/sleep-cycle-alarm-clock/id320606217?mt=8

Local Concerts

Local Concerts shows you the bands that have shows near your location. It has saved me from missing many shows this summer. 

http://itunes.apple.com/us/app/local-concerts/id315472348?mt=8

Gowalla

As location is growing on the web, Gowalla still continues to impress me. Their app design is beautiful and the service gives you reason to get out of your house and discover places you normally wouldn't. 

http://gowalla.com

Desktop Apps

Dropbox

I'm tired of reading blogs posting about how useful Dropbox is, but if you aren't using it yet, please slap yourself. 

http://dropbox.com

Isolator (Mac OS X only)

Isolator is an app that helps you concentrate on the window you are currently using. 

http://willmore.eu/software/isolator/

PhoneView (for iPhone)

Phone view is way more powerful than the current iTunes sync. It allow you to view and save your call log, text messages, and use your iPhone as a USB drive. 

http://www.ecamm.com/mac/phoneview/