This are the plugins that I find very useful and will not harm your Wordpress Installation because nowadays there several Wordpress plugin released which can harm your Wordpress because of it’s messy coding, I have used this before so I can assure you that this plugin are really safe to use. Just follow the instruction given by each plugin in able for you to execute it on your site successfully.

• Simple Tags
  Is the perfect tool to manage perfectly your WP tags.
• Category Tag
  If you want to show your categories in a cloud like structure this is a right plugin for you.
• Paged Comments
  This is a very useful plugin especially if you get a lot of comment on your blog because it breaks your comment into number of pages.
• Theme Switcher
  It allows your reader to switch among your installed themes in your “wp-content/themes”
• Gravatars 2
  It display your commenters Gravatar (globally recognized avatar).
• WP Cache
  WP plugin that speed up the load of your page because it helps you cache your Worpress pages and storing them in a static file for serving future requests directly from the file rather than loading and compiling the whole PHP code and then building the page from the database. It is a very useful plugin if you have a high traffic site or low performance server.
• WP-Lightform
  is an AJAX / PHP contact form with spam protection, form validation and custom form elements.
• Riffly
  WP plugin that in able your commenters to post their comments via video.
• Lightbox 2
  A pop-up effect to open your images by using JavaScript.
• Exec-PHP
  It allows to execute your codes in php in the pages of your Wordpress.
• TTFtitles
  It allows you to use your own customized fonts in the headings of posts, pages, to sidebar, etc.
• Twitter Tools
  It shows your updates from your Twitter, it allows you to make updates in your wp panel and to transform updates into posts.
• Intouch Ajax contact form
  It works perfect and filters all the spams coming. It also allows you to customize the email subject, thank you messages, and adds addition form fields.
• Secure and Accesible PHP Contact form
  This easy-to-install form has many accessibility, usability, and security features and also one of the top rated plugin.
• Bub’s Button Board
  It’s a customizable plugboard for your Wordpress blog.