I pointed out that multiple plugin support is a factor of a popular WordPress theme, in a previous post. Here’s a list of popular plugins your theme should support.

• Widgets – Allows you to easily rearrange sidebar blocks and turn them on or off.
• Contact Form – Self-explanatory.
• Spam Karma 2 – Filter comments for spam and keeps count of how many spam comments. In most cases, this plugin will break your theme via the wp_footer() function. You need to test this plugin and style it specifically for your theme.
• Optimal Title – Place the blog title after the individual page title. Instead of Your Blog Title > About, this plugin changes it to About | Your Blog Title.
• Related Posts – Generates a list of links, possibly related, to a certain post. It’s basically an unordered list without the UL tags.
• Adsense Manager – Allows bloggers to easily integrate and manage Adsense ads.
• Ultimate Tag Warrior – A post-keyword tagging system that allows you to organize posts by keywords with or without the normal categories.
• flickrRSS – Displays photos from flickr.com to your blog.
• MyAvatars – Displays readers’ My Blog Log avatars in the comments template.
• WP-PageNavi – Allows you to browse from page to page by numbers instead of Next and Previous.