This checklist is useful when you're launching / relaunching a website or webapp. It doesn't substitute for a full QA; it's more of a 'don't forget to…' list.
Pre Launch
Verify that QA is complete and there are no outstanding issues / tickets.
Verify server settings. Vhosts must be setup and ready. The domain must be pointed at the new server (or have a 300s TTL if you're still pointing to the old server until launch).
Make sure you've migrated the old .htaccess file, including special settings and any redirects included there.
Do a final spot check of pages or your app (preferably in IE) and make sure nothing looks broken; all pages have title tags, meta data, etc.
Make sure Google Analytics is installed, including any advanced tracking (virtual pageviews, e-commerce, event tracking, etc.)
Launch
Make sure you have a written launch procedure, as well as a rollback procedure. These can be simple - a few steps written on a napkin for the launch, and just a 'restore backups' step for the rollback. But make sure you actually have them, and have them written down. Launch is no time to ad-hoc important system changes.
Things you'll probably want to include on the launch plan:
Updating/installing any needed server side software (e.g. an updated version of curl)
Pushing out your actual code.
Making specific file or filesystem changes needed for launch (e.g. creating a needed symlink or changing a specific config file)
Running database update scripts.
Running one-time scripts, e.g. a permissions fixer script or something.
Setting up needed cron jobs.
Making any needed changes to Google Analytics goals, etc. as you go live.
Post Launch
Spot check the website again, probably in IE, to make sure everything looks good. Also spot check functionality (e.g. registration, login, contact forms, basic functionality in your app).
Make sure httpd access and error logs are going where you think they should be. Review the files and make sure no unexpected errors are showing up.
Spot check a few redirects (e.g. from old pages) to make sure they work.
Set TTLs back to 86400.
Check Google Analytics and make sure you're still getting tracking data and goals, etc. are still working. Note that Google typically takes ~3 hrs to update data.
Make sure any monitoring you've setup is showing everything is online and operational.