Zynga, the maker of FarmVille and other Facebook games, is expected to file for its initial public offering tomorrow or shortly thereafter, seeking a valuation of $15 billion to $20 billion. It's one of the tech bubble's most rational IPOs.

Although Zynga's business is selling imaginary products in virtual worlds — seed corn to FarmVille players, weapons to Mafia Wars gamers — its profits are very real, expected to reach $630 million this year on $1.8 billion in revenue and up strongly from 2010. That makes it a slightly less insane investment than moneypits like Pandora and Groupon — at least until frenzied stock buyers bid it into the stratosphere.

[Photo of Zynga CEO Mark Pincus via Getty]