I've certainly been asked this question more times than I would have ever thought possible.
Two parties decide to begin a joint venture, and they're such good friends, they do it on a handshake.
Fast forward two years later, and things haven't gone that well; party #2 has taken over the books, and won't even let party #1 see them. (Doesn't that raise an awful lot of red flags?)
At that point, party #1 decides to contact a lawyer to see what his rights are, figuring he probably has none, as they never entered into a written agreement.
It turns out, he's wrong. And, quoting from some New York caselaw, here's why:
"The statute of frauds is generally inapplicable to an agreement to create a joint venture (F.S. Intertrade Off. Prods. v Babina, 199 AD2d 95, 96 , lv denied 83 NY2d 757 ) or partnership (Prince v O'Brien, 234 AD2d 12 ;Rella v McMahon, 169 AD2d 555 ). This is because, absent any definite term of duration, an oral agreement to form a partnership or joint venture for an indefinite period creates a partnership or joint venture at will (see Shandell v Katz, 95 AD2d 742, 743  [partnership at will may be dissolved, without liability for breach of contract, on a "moment's notice"];Alnwick v European Micro Holdings, Inc., 281 F Supp 2d 629, 644 [ED NY 2003] ["Where . . . there is no definite term of duration for the joint venture, it may be terminated at will"]).