https://wordpress.org/plugins/{plugin name}

dmsguestbook
Simple guestbook. Not developed anymore, but it contains everything needed so I like it. Stores posts into database, supports styling, removing of spam. Supports akismet.

akismet
Spam blocker used with dmsguestbook. Advantage: your customers do not have to deal with captcha. In my case it lets through a little of spam, about 1 of 100. But it is helpful and it is worth the money.

backwpup
Backuper of everything on site. I have scheduled weekly backup of db + some files which is then sent to my e-mail as zip, and sometimes I run a job that backups whole site into zip which I download later on.

count-per-day
Shows visitor statistics. Visitors on single pages, searched queries on google that led to your site, etc.

lightbox-plus
Nicer displaying of images in galleries.

list-category-posts
[shortcode] to show posts from specific categories in a way needed. Something like showing list of blogs, but in customized way.

post-expirator
On posts you can set a date when the post changes category.

rs-event-multiday
You can add an event date to post, and then show list of events in widget with dates and links to post.

subpage-view
[shortcode] to shows a list of sub-pages.

tinymce-advanced
Upgrade to text editor.

ultimate-tinymce
Upgrade to text editor.

wp-cycle
Scrolling show of images, for adverts.

wp-filebase
File manager where you can add files and then show them on web. Handles downloads, counting downloads, showing the file list, folders, templates for displaying files, nice links to files etc.

wp-jump-menu
Easier navigation in pages and posts when editing web. You can jump to post or page by name.

wp-php-widget
Widget containing any php code.

Post By Email
Create new posts on your WordPress blog by sending email to a specific email address on your server.

google-maps-v3-shortcode
Shows google map on the page. Plugin is old and not maintained. I don’t like it much but it works.