Two of the best apps for this are CloudApp and Skitch.
Cloud app has an option to automatically upload screenshots (image courtesy of Softpedia). It is very drag and drop friendly and pastes the URL to your clipboard for convenience.
Skitch is a little more heavy on the screen snapping controls. It's like grab on steroids. You can quickly hide private information, point out items, and edit the snap before uploading.
The free version of both are well worth it. Both have paid upgrades that are low cost, but you don't need to pay to use either. I have also heard really good things about Droplr so you might check it out as well.
Both CloudApp and Skitch are available on the App Store. The two are better together - skitch is better for grabbing a great screen shot, and droplr for upload and sharing it.