Where I'm from it's to be added to the end of a phrase or sentence in order to give it romantic implications in IM and texts.
Originating from the MSN eye-brow waggling emoticon which is sometimes has the keyboard shortcut "#"

Used as a tagging marker in Twitter for creating lists and groups.

A grid for the playing of the game Tic Tac Toe.

a number sign. mostly used in the internet and when texting. Instead of typing the word "number", you can use this as an alternative.

The official symbol of attention seekers on the internet

the waffle key. many people use it as an abbreviation for 'number.' it looks like a waffle.(:

A written symbol for the term "waffle" meaning one who has homo-sexual tendencies or is prone to homo-sexual actions; Represented through hand gestures by placing two fingers of one hand (as in a peace sign gesture) in one orientation and placing two fingers of the other hand behind the first hand and oriented perpendicular to the first hand, thus forming a visually analogous hash mark with the hands;