From the official Google Blog we find the post All buttoned up which informs us that

As the Google Toolbar has gotten more popular, the greatest source of ideas about new features has come from our users. The breadth and variety of these requests is so large that it's hard to satisfy everyone. But then we started noticing engineers on the team had cool hacks on their Toolbars for doing customized searches on our internal bugs database, corporate employee directory, etc... We were barely done asking ourselves whether it was possible to offer this capability in the new Google Toolbar beta when one of the engineers started designing a feature called Custom Buttons. Here are some of the coolest aspects of Custom Buttons and why I think they're a big deal:

1) Simple API: The term API is almost a misnomer -- it literally takes seconds to make one of these. I just can't resist the urge to make a new one every time I run into new website. A couple of simple steps and voila - a new button's sitting on your Toolbar (check out the Getting Started Guide).

2) Flexibility: The simple inclusion of RSS & Atom feeds (and particularly allowing the update of toolbar button icons through feeds) has allowed for buttons like a weather button and a mood ring button.

3) Accessibility: Most users don't even need to make buttons. It takes one click on our buttons gallery or on a website that offers them to install a button for your favorite sites. And the custom buttons we built to search our intranet showed us how valuable a customizable toolbar can be to organizations, so now there's an enterprise version of Google Toolbar that can be securely deployed across a company.

I use the Google toolbar quite frequently when performing searches and one of my biggest gripes is that it doesn't give me the option of using Google Music Search for my searches. So when I found out about the new version of the toolbar, I downloaded it out and clicked on "Add Search Type" which took me to the Google Toolbar Button Gallery. Guess what? There's no option for adding Google Music Search to my list of default search types.

So I tried reading the documentation on Getting Started with the Google Toolbar API so I could figure out how to add it myself and came up short.  The entire API seems to assume that some stuff gets installed in my right-click menu in Internet Explorer which doesn't seem to be the case. I wonder if I need to reboot to get the changes to show up? Bah. Now I feel irritated that I just wasted 15 minutes of my time on this.