ThatsBlog.com

Adding a bookmarking button in your blog has many advantages:

1. Easy to Bookmark and Share: Bookmarks can be addded to a particular social bookmarking service easily with a click.
2. Promote your Blog: Its a good tool to get you traffic.
3. SEO: It acts as a great way to increase the search engine rankings.

It also has a drawback. If your blog is facing bandwidth related issues then, adding these buttons can make it worse.

I found the site addthis.com quite good as it has many features such as free statistics and its quite easy to install and customize.

Steps to add addthis button in a blogger blog:

1. Create an account with addthis.
2. Then, create your button. You have the option to change the look of the button.
3. You can make this option “Where? Where do you want to put the button?” as “on a Blog”.
4. Select the blogging platform as “Blogger”
5. You will get codes for the drop down and original buttons.
6. If you want to include the button below each post, login to your blogger admin area, select Template then Edit HTML, then copy-and-paste the code below into the template, right after the <p class=”post-footer”> tag in the classic template.

For example, I have added a drop down addthis button in my test blog: http://blogforthatsblog.blogspot.com