Recommended Plugins

Plugin Name	Plugin Description
Must haves
All in One SEO Pack	Great SEO plugin and probably the most widely used for WordPress sites. Enough said.
Google Sitemap Generator	Again, another plugin that I recommend you use, along with the All in One SEO Pack, as they do different things. This plugin recently just got better as it now also adds your Custom Post Types to your site's sitemap.
Block Bad Queries	This one adds some extra security for your WordPress site.
Login Lockdown	Should be pretty self-explanatory. Adds additional security for the WordPress login form.
WP Super Cache	This plugin generates static html files from your dynamic WordPress blog. After a html file is generated your webserver will serve that file instead of processing the comparatively heavier and more expensive WordPress PHP scripts.
W3 Total Cache	W3 Total Cache improves the user experience of your site by improving your server performance, caching every aspect of your site, reducing the download times and providing transparent content delivery network (CDN) integration.
Aside	These are probably the two most well known optimization plugins for WordPress. I recommend you do your own research regarding them, as the efficacy of them at times can come down to your setup and other factors.
Project specific
Contact Form 7	I haven't used this one in awhile (as I typically use Gravity Forms, below), however I recommend using this if you don't feel like paying for a premium solution, or hell, just roll your own contact form if you feel like it.
Back Up WordPress	I recently used this on a client's project and liked it. Mainly I liked that it seemed simple and straightforward and backed up not only your theme and plugins, but your database as well. It's hard to find a good, all-encompassing backup solution for WordPress. Of course you could go premium and spring for VaultPress (below).
WP DB Manager	I've used this plugin off and on, but I've found it depends on your hosting company's setup on whether this plugin will be of use or not. If it does, this plugin is great because you can schedule automatic database optimization and backups. It is important to note that this plugin doesn't back up your theme files however.
Posts 2 Posts	This is an amazing plugin when you'd like to create relationships between posts, pages, or custom posts and pages. For example, I've used this plugin in the past for shopping area site to relate and show various features and specials to the proper restaurant or shop–freakin awesome!
Aside	As an aside to the backup plugins, I do always try to setup automatic backups for my clients, whether through using a plugin or creating a couple of cron jobs to do the trick (which also allows you to skip utilizing a plugin for that–the less plugins you use the better)
Premium plugins
Gravity Forms	This one has become my go-to plugin for contact forms (but can be used for much more). It is an awesome, extensive, and highly customizable premium WordPress plugin.
WordPress Events Calendar Pro	I can't recommend this premium events plugin highly enough. I've used it on several occations now, and it really is my go-to plugin for anything events-wise or when I need a calendar. Just about every other WordPress events/calendar solution I've tried is appalling, and Modern Tribe gets it right.
VaultPress	I actually haven't gotten the chance to use this plugin yet, but have used different solutions for automatic backups not necessarily liking anything I've used. I figure you can't go wrong with using VaultPress for your site backups as this is made by Automattic, the creators of WordPress.