I've just completed the upgradation from WordPress 2.0.7 to WordPress 2.1, and found my WordPress theme "Connections" (highly modified by self) is fully compatible with WordPress 2.1.

Here is the list of my plugins that are compatible with WordPress 2.1:

1. AJAX Comments 2.07
2. Addicted To Live Search 1.02
3. Adsense-Deluxe 0.8
4. Ajaxified Comment Preview 0.1 beta 1
5. Akismet 2.0 
6. AuthImage 1.1
7. Bad Behavior 2.0.10
8. Customizable Post Listings 1.1
9. Enhanced Post List 0.4 
10. Feedburner Feed Replacement 2.2
11. Giraffe AJAX Calendar 2.1
12. Google Sitemaps 2.7.1
13. Inline Ajax Comments 0.3
14. KG Archives 2.9
15. LMB^Box Comment Quicktags 2.4
16. LMB^Box Smileys 3.2
17. Landing sites 1.31
18. MyBlogLog Widget 2.0
19. Pagebar 1.2
20. Pierre’s Wordspew 2.91
21. Post Updated R1
22. Quote Comment 2.0
23. Related Posts 2.02
24. Share This 1.4
25. Snap Preview AnywhereTM Plugin 1.1
26. Subscribe To Comments 2.1
27. Ubernyms 2.6
28. Ultimate Tag Warrior 3.1415926
29. UtterQuotes 1.0.2m
30. Viper’s Plugins Used 1.12
31. WP-ContactForm 1.4.4
32. iG:CoComment 0.5
33. iG:Syntax Hiliter 3.5
34. the_excerpt Reloaded R1
    

    Plugins released for WordPress 2.1:

    1. WP-DBManager 2.10
    2. WP-PostViews 1.10
    3. WP-UserOnline 2.10

Alternatley, you can also refer to Codex.WordPress page for WordPress 2.1 compatible plugins.

Good Luck!