For my poor memory,
1.- Contact Form 7 – Contact Form 7 can manage multiple contact forms, plus you can customize the form and the mail contents flexibly with simple markup.
2.- Force Regenerate Thumbnails – Useful if your site already have a lot of content and images. Allows you to delete all old images size and regenerate the thumbnails for your image attachments.
3.- Acunetix WP Security. Useful for security checks.
4.- W3 Total Cache, improve time response.
5.- Smushit, compress image files.
6.- WPTouch, adapts the look & feel to the different mobile formats.
Updated in 2015
7.- WordPress SEO by Yoast, for improving the SEO of each post.