Here is a list of 21 indispensible plugins. Below are my suggestions, based on [IMD] Internet Marketing Design:

1. Google Integration Toolkit (this is the simplest option that gets the job done)
2. Google XML Sitemap
3. Smart YouTube (Vipers Video Quicktags if you want more options)
4. Quotes Collection for Random Elements (testimonials, did you know, random facts, or even quotes:  the widget hack helps)
5. Calendar (Google Calendar if you want to embed offsite)
6. Newsletter (Simple, gets the job done, and if savvy, can be hacked)
7. Email login (once a user creates an account with unique username/email, the user can login with the username or email).
8. root Cookie (hijack your WP cookie across a subdomain to piggy back off of WP for authentication).
9. SimpleModal Contact Form: Can be hacked to customize the colors, and AJAX contact form.
10. WP-Forum:  simple is good
11. WP-UserOnline
12. Audio Player (although you can’t place in the sidebar, it only hooks using add_filter on “the_content()” you can place in a draft page, grab the code for a specific link, and place wherever you want)
13. Optimize performance [2] using WP Super Cache and PHP Speedy Web Optimizer and Speed Cache.
14. Breadcrumbs by Yoast
15. WP-Polls (hacked the database to allow for HTML in Q/A (Hacked polls-admin-js.js to change text input to text area :: type=”text” size=”50″ maxlength=”200″ name=”polla_answers_new[]“; likewise, changed lines 188,216 of polls-manager.php)
16. BM Custom Login
17. Category Order
18. WP Cumulus
19. TinyMCE Advanced (to prevent the WordPress Nazis from removing, stripping out your <BR /> tags)
20. Track Users Online