it's a borderline intimate thing, although I might try it with close female friends (in the platonic sense) - I've know the friend I'm thinking of for about 15 years - and family (my cousins, sisters-in-law, etc for example). it depends on the individual, and their likely reaction.
so - a guy might tickle a girl he thinks of as part of the family; and he might also do it as a "try out" for physical contact. guys like physical contact & will make attempts at it which include tickling & play-fighting amongst others.
In my experience, tickling is standard operating procedure in the Show-Him/Her-I-Like-Them-Without-Having-to-Come-Out-and-Say-It playbook. Unless you've been friends for a very long time, or have already been through/discussed mutual romantic interest, I'd say tickling is a pretty clear indicator of attraction.
There are, of course, exceptions: girls who are, by nature, particularly physical or touchy may see tickling as harmless, and do it without any sort of ulterior motivation. As guys we have to be wary of this, as we may interpret it as some sort of signal and then end up in a whole mess of trouble. There are also guys who are the same way: naturally touchy people. This is much rarer for men though, so I'd say that any guy who tickles is either romantically interested or a pig who is looking for an excuse to touch women.
As general advice, I say this: no matter your gender, I'd say don't tickle someone unless you're romantically attracted to them.
it depends on the relationship... if you are close friends then it could be like a brother and sister thing but he could also be trying to bring your friendship to another level (possibly dating) if you are not "friends" with him but you see that he is interested in you in other ways AND he is tickling you, then he could really just be trying to have some physical contact that will lead to intimate physical contact (touching higher or lower/ kissing)
i have had just a guy friend tickle me.but he and I both kno there is nothin there but a really good friendship.so yea I have had jus friends tickle me.but also if it is not a guy tht has always done it.then it deff. is a way to show you he is interested.and jus can't find the words to say it.so he is showin it!