The first one is the one you want:
There are many, many ways to access your files in Aperture: You can use any Open / Attach / Browse dialogue. On the left there's a Media heading, your pics can be accessed there. Command-Click for selecting multiple pics.
(Note the above illustration is not a Finder Window. It's the dialogue you get when you go File -> Open)
You can access the Library from the New Message Window in Mail:
There's a similar option in Outlook and many, many other apps. If you use Apple's Mail, Entourage, AOL or Eudora you can email from within Aperture.
If you use a Cocoa-based Browser such as Safari, you can drag the pics from the Aperture Window to the Attach window in the browser.
I use Blogspot. The method Terence described works very well for that. From Blogspot you can add one or two photos by just clicking on the add photo icon and then browse to the picture.
For a group of pictures I use Picasa Web Uploader. This allows me to add a link to a slide show. Picasa then stores the pictures and links to it from the blog, or plays right in the blog, if you prefer.
Using Picasa (not Picasa Web Uploader) seems like a duplication of effort to me.