I've never used any before - never needed to, but now that my goatee is getting really long I do feel it is something that I should get.

Irish Dan has a webshop that sells some that everyone here raves about. http://jefffsbeardboard.yuku.com/topic/28461/IrishDan-20-months-post-1344 - if you scroll down a little, you can see his signature in some of his later posts. I've got some on the way over here, but it'll be a couple of days before it is delivered I guess.

Others have suggested jojoba oil (health shop) and coconut oil (supermarket) as a cheap and easy oil. Not tried any myself, but I have seen plenty of people suggest it.

Oh, and if you buy from Dan, you can enter BEARDBOARD when you check out and you get 10% discount.