There doesn't seem to be a way to indicate that one only likes a person with whom one is friends, rather than loves them. Is there a way to indicate this while still referring to the friend as 'tu'?
Je t'aime bien (literally: I love you much).
To change “love” to “like”, you need to modify it with an adverb.
Je t'aime bien like Andrew Lewis suggests will work fine too, but it is safer to use Je t'apprécie with same sex persons.
In fact, to like is often translated as apprécier in French.
"I like you" can be translated by:
Je t'aime bien. ( I like you )
Je te trouve sympa. ( I think you're nice )
Je t'adore. (I like you a lot. (More used among girls))
T'es sympa. ( You're nice )
Je t'aime beaucoup. (I like you a lot, (and let's stay friend :P) )
T'es un super pote. ( You're a super nice friend )
On s'amuse bien ensemble. (We have fun together)
Generally, one would use:
Je t'aime bien.
However, if you want to be more formal, then use rather the following:
If you speak French with a foreign accent, then I suggest you use the second sentence as it sounds less naïve than the first one, which could be directly taken from the first lesson of any learning course.
More casually, "Tu es sympa"
You can even say "Je t'aime", but you need non-verbal communication like intonation and gestures, or some context or background with the other person that can avoid any misunderstanding.
Another good expression Tu me plais.
Friends usually address themselves with "Je t'adore" or also you could make périphrases like "J’apprécie beaucoup être [en ta compagnie|avec toi]".
I like you can be translated to 'je te kiffe' as a familiar language. Its origin is Arabic, see kiffer on the Wiktionary.
As other said, you are dealing with an untranslatable term. Languages support cultures, and different cultures build different social relations.
In French, there's no equivalent to like, just like there's no equivalent to vous in English. Of course one may use different language register in English, as well as one may express a whole shade of relational feelings in French. But it will be reflected in more complex constructions that you can't sum up in a one to one locution translation.
Using tu is already a way to express a more intimate relation. If you want to express that you like to spend time with a friend, you may say "je suis heureux qu’on soit ami(e)s, tu sais?". But as said, you will need to be more specific on what you want to express.
Edit by PamCam: this is off topic, but English you actually corresponds to French vous as can be seen from the verb it takes i.e. you are and not you is; therefore it is more appropriate to say that standard English does not have a form that corresponds to the French form tu. Some dialects, though, allow expressions such as you is or yous, but that is a different matter altogether.
Addition by user 168676: I agree with PamCam and can mention additional information corroborating his/her contentions. In ancient times the English language has had the equivalent of a second person singular; the subject pronoun was "thou" in which we find the t of "tu" in french and a phonetically close sound in the German "du" which is modern German "tu"; the second person of the verb "to be" was "art" in which again we find the t of the second person singular of the German "sein" which is "bist" (verb "to be" in German); the English language is a Germanic language, that being due to the early migrations towards Britain of a germanic people known as the Angles. It might be added that the second person singular has persisted in English poetry for a long time and that it is found in some English dialects still spoken in England; we get a substantial exposure to one of those dialects in the contemporary (and once controversial) literary work of D.H. Lawrence, Lady Chatterley's lover.
Whether addressing the other person as "tu" or as "vous", "I like you" can always be rendered without ambiguity by "J'ai de l'amitié pour toi." and "J'ai de l'amitié pour vous.".
"Je t'aime bien" est ce qui se rapproche de le plus de "I like you".