There are so many wordpress plugins which are free to download. There are more then 21500 wordpress plugins till date of jan 2013. There are several nice plugins also which we need to use.
You can refer following articles.
- Best wordpress plugins for wordpress security and protection
- wordpress plugins for stats or statistics
- 16 Best WordPress Plugins which every WordPress Blog Should install
If you use many plugins then slow down the site
You have more then 30+ plugins then that plugin will execute the extra sql quries while opening each and every page. Depends on your wordpress plugins which is installed on your site. your site load slowly.
Some free plugins will ad the unnecessary code in your site
Some wordpress plugins are not safe to install
Some wordpress plugins are not secure to install in site. It may cause the security issue. Some plugins are easy target for hackers. You should read the some review about plugin before start using in your site.
Some wordpress plugin will cause issues in your site
Before installing the wordpress plugins you should create development or test site and on development site you should first try to install the new wordpress plugins. After that you should install on current site. Some plugins will cause the jquery conflicts in your site.Do not use many wordpress plugins in website for SEO