The Galaxy S2 comes standard with their Memo app, which you'd basically use to make quick notes:
It works ok, but I'm not too fond of it. I decided to look for an alternative, and found Evernote - the nice thing about Evernote, is it includes the ability to take a quick picture or video or even voice recording, as well as traditional text notes. Also, a very handy widget is available.
Furthermore, you must create an online account, which means your notes gets synchronized with the Evernote website, so you can access your notes from pretty much every where.
It works a lot better than the default Memo app that comes with the phone in my opinion, and I'd highly recommend getting it as soon as you can, it will really help when you need to make a note of something real quick :)
Get it here