As far as a WordPress site goes, these two plugins are a must. I use them on every project and they work flawlessly.
The first is Google XML Sitemaps from BestWebSoft (http://bestwebsoft.com/plugin/google-sitemap-plugin/). Your website will need to generate a Sitemap to help search engines index your content. Site maps are fine if you’ve got the time to spend on your native code (PHP if you’re using WordPress) and are happy playing about with XML, I’m fine with those technologies but for speed of deployment there’s no way your going to beat this plugin. The Sitemaps this plugin creates are updated every time a blog post is added, so it’s all automated. Once the initial Sitemap is created it’s just a case of submitting the file to Google Webmaster Tools for checking, this always works first time. Awesome.
The second is W3 Total Cache by W3Edge (http://www.w3-edge.com/wordpress-plugins/w3-total-cache/). WordPress sites can be heavy. There’s all manner of plugins, all of them bringing their own functionality to the party in the same way that a new football manager will turn up with new backroom staff. As a developer, looking at the source code for a WordPress site can be counter intuitive, the eye watering volume of accompanying files goes against everything we try and achieve with regards to front end tuning. Without going into too much detail, most of what this plugin is about is minification and caching, both of which are handled expertly. On deployment the site instantly becomes snappier with almost instant second page views. Beware though, the caching can be quite severe so any updates to front end styling will need to be accompanied by a cache empty, this is easily handled by the plugins additions to the WordPress interface. Awesome again!