For every new project:
silverstripe/googlesitemaps
kinglozzer/metatitle - Puts back the page Metadata fields
purplespider/mypswd-tweaks - A bunch of my own tweaks
axllent/silverstripe-email-obfuscator - Automatically obfuscates e-mail addresses entered via the CMS (to hide them from spammers)
kinglozzer/htmleditoruploadfield - Allows you to upload files directly in the Insert Link box. Hopefully will be part of the core soon.
I also use these a lot of the time:
silverstripe/staticpublisher - For basic sites. Dramatically increases page load speed.
unclecheese/betterbuttons
undefinedoffset/sortablegridfield
jonom/focuspoint - Let's the CMS user pick the part of an image to crop around (rather than just the centre).
jonom/silverstripe-betternavigator - Mainly so it's obvious to CMS users when they are viewing the Draft or Published site outside of the CMS.
axllent/silverstripe-scaled-uploads - Automatically resizes images uploaded to a maximum size. Stops the hosting account filling up when CMS users upload loads of high-resolution images, which are only ever displayed at a smaller size.