Formal:
* For your information (this is the full version of the acronym)
* Please note
* Please be advised
* To inform you
* This is to let you know
Informal:
* Heads up
* Just letting you know
* Thought you might want to know
* In case you didn't know
* Just a quick note
* Quick update
More specific:
* Just wanted to confirm...
* Regarding...
* With regards to...
* As a reminder...
The best alternative will depend on the specific situation and your relationship with the recipient.