What kind of spider is this?!?

These are called banded Argiope garden spiders. Quite harmless:

http://www.youtube.com/watch?v=m2b4NO8Aj04

They are a beneficial garden spider, and there is certainly no need to kill them.

Here is one by my drive last season, you might note the male with her:

http://www.flickr.com/photos/32083154@N02/61662671...

These spiders often spin a zig zag pattern in their web called stabilimenta, and looks like they may be writing in their web. As such they are sometimes called writing spiders.

http://www.walterreeves.com/insects-and-animals/ar...

ps; Nice photos.