i use Skitch to capture hundreds of screenshots a day at work. I work in QA capturing bugs in software. needless to say its the greatest screenshot capturer ever.
1. find bug
2. cmd-shift-5 to capture
4. draw and write on it
5. click webpost or just drag skitch into email
7. paste URL into bug ticket or just click send email
Jing is a close second (and does great video) but skitch is still faster and more pleasant to use.
Realmac Software (RapidWeaver) recently released Little Snapper, which is similar to Skitch. I haven't thoroughly tested it yet, but from what I've seen so far, it's pretty cool.
Skitch also works well as a simple drawing program. That's actually the main reason I use it. I just launch it, I have a large blank canvas and I draw.