A curated list of useful WordPress plugins
Custom Post Type UI
Custom Post Type UI provides an easy-to-use interface for registering and managing custom post types and taxonomies for your website.
There are a heap of plugins for managing custom content types and taxonomies, but this is one of my favourites. It's got a slick interface, is well maintained and updated, and generally does just what you want with the minimum of fuss.
Unfortunately WordPress does not have the best track record with regards to security, and WordPress sites are often the victim of hacking attacks. One of the easiest things you can do is to install a security plugin such as iThemes Security - which will quickly and easily harden your WordPress installation against many of the common attack vectors. There is a free version and a paid-for Pro version, but even the free one provides a significant upgrade of your site's security.
WordFence is similar to iThemes Security, but it takes a slightly different approach. WordFence sets up a Web Application Firewall (WAF) that hooks in to the PHP request process, allowing it to intercept malicious requests before they even reach the WordPress application code. This makes it a pretty powerful line of defence, and combined with its global "Threat Defense Feed", which automatically provides the plugin with the rules for combatting the latest attacks, it stops a significant number of otherwise troublesome attacks.
Secondly, WordFence comes with a site scanning facility, which scans the files in your WordPress installation and compares them to the original copy of the file in a clean WordPress installation. This means it can quickly and easily give you a list of files which have been modified, and this is amazingly useful in cleaning up after a hack, and of course for peace of mind in the first place.
This plugin provides several useful features, including caching, minification and other performance related functions. However, for me one of the most useful things is it analyses your database, and reports on unused data and tables.
This fixes a common headache with many WordPress installs - by the time your site is of any decent age, a lot of different plugins have been tried, tested and uninstalled - and most plugins do not provide an option at uninstall time to remove the database tables that they have created, and generally clean up properly after themselves.
This means that over time your WordPress database can grow in size, and be full of useless, unused tables and data. Once of the best things you can do is to keep on top of this problem, to improve performance and reduce the chance of malicious data being stored in the database - but it's very tricky to do this manually. This plugin just makes it very easy and reports which plugins have created each table, whether they are still installed and active or not, and allows you to remove the tables with a press of a button.
WP Migrate DB
This is a very powerful tool, that assists you in moving your WordPress site from one server to another. You can export your database, but crucially at the same time you can perform a search and replace to convert, for example, your old domain name to a new domain name, or one filesystem path to the new path on the new server.
Secondly, it allows you to perform ad-hoc search and replace functions directly on the live database, which is pretty handy - of course you should use the backup function before doing any direct editing of the live database.