Plugins

Jetpack: statistics, forms, social sharing, galleries, and more
https://wordpress.org/plugins/jetpack/

Forms

Gravity Forms (paid) http://www.gravityforms.com/
Ninja Forms https://wordpress.org/plugins/ninja-forms/ & http://ninjaforms.com/

SEO

WordPress SEO by Yoast https://wordpress.org/plugins/wordpress-seo/
All in One SEO Pack https://wordpress.org/plugins/all-in-one-seo-pack/

Images

Simple Lightbox https://wordpress.org/plugins/simple-lightbox/
MetaSlider https://wordpress.org/plugins/ml-slider/ & http://www.metaslider.com/

Site Speed

Caching

WP Super Cache https://wordpress.org/plugins/wp-super-cache/
Quick Cache https://wordpress.org/plugins/quick-cache/
Hyper Cache https://wordpress.org/plugins/hyper-cache/
W3 Total Cache https://wordpress.org/plugins/w3-total-cache/

Minifying

Autoptimize https://wordpress.org/plugins/autoptimize/
Better WordPress Minify https://wordpress.org/plugins/bwp-minify/
W3 Total Cache https://wordpress.org/plugins/w3-total-cache/

Security

Wordfence Security https://wordpress.org/plugins/wordfence/
Sucuri https://wordpress.org/plugins/sucuri-scanner/
iThemes Security https://wordpress.org/plugins/better-wp-security/

Backup

BackWPUp (free & paid) https://wordpress.org/plugins/backwpup/
https://marketpress.com/product/backwpup-pro/
Akeeba https://wordpress.org/plugins/akeebabackupcore/
UpdraftPlus https://wordpress.org/plugins/updraftplus/
Vaultpress (paid) http://wordpress.org/plugins/vaultpress/
BackupBuddy (paid) http://ithemes.com/purchase/backupbuddy/

Migration

All in One WP Migration https://wordpress.org/plugins/all-in-one-wp-migration/
WP Migrate DB https://wordpress.org/plugins/wp-migrate-db/ & https://deliciousbrains.com/wp-migrate-db-pro/
UpdraftPlus https://wordpress.org/plugins/updraftplus/
BackupBuddy (paid) http://ithemes.com/purchase/backupbuddy/

Themes

StudioPress (theme store) http://www.studiopress.com
Quark https://wordpress.org/themes/quark/ & http://quarktheme.com/
Spacious https://wordpress.org/themes/spacious/ & http://themegrill.com/themes/spacious/

Articles

Moving WordPress http://www.jasonbobich.com/web-design/moving-wordpress-to-a-new-server/

Learn to code

Codecademy http://www.codecademy.com
Team Treehouse http://teamtreehouse.com

Code references

Sitepoint: HTML, CSS and PHP reference and tutorials
http://www.sitepoint.com/
http://reference.sitepoint.com/
CSS Tricks http://css-tricks.com/almanac/

WordPress Docs

Template Tags http://codex.wordpress.org/Template_Tags
Conditional Tags http://codex.wordpress.org/Conditional_Tags
Hooks (actions) http://codex.wordpress.org/Plugin_API/Action_Reference
Filters http://codex.wordpress.org/Plugin_API/Filter_Reference

Software

Dash: Documentation browser and snippet manager (Mac)
http://kapeli.com/dash

Code Editors

Coda (Mac) (paid) https://panic.com/coda/
Notepad++ (Win) (free) http://notepad-plus-plus.org/
Text Wrangler (Mac) (free) http://www.barebones.com/products/textwrangler/
PhpStorm (Win, Mac, Linux) (free & paid)
http://www.jetbrains.com/phpstorm/

FTP Clients

FileZilla (Win, Mac, Linux) (free)
https://filezilla-project.org/
Transmit (Mac) (paid) http://panic.com/transmit/
Cyberduck (Win, Mac) http://cyberduck.io/