RockYou, widget maker, is the latest example of a sane valuation heartbreaker, as it is undertaking efforts to secure an investment from mainstream financing firms that would value the company at between $300 million and $400 million.

First reported by Valleywag last night, the start-up, said one source, “is being squired around Wall Street” by investment behemoth Morgan Stanley (MS), in search of the same kind of deal its rival Slide got in January.

BoomTown broke the news of that deal, which nabbed Slide $50 million and a $550 million valuation with investments from blue-chip investors T. Rowe Price (TROW) and Fidelity.

Thus, RockYou’s motto: Anything Slide can do, we can do slightly smaller!

And, indeed, not to be SuperPoked by Slide CEO and Founder Max Levchin, sources said RockYou Co-Founders Jai Shen (also CTO) and Lance Tokuda (CEO) were quickly on the march for their own payday.

It is, in fact, a quest that a lot of Web 2.0 companies seem to be on, since the sector’s fearless leader–Facebook–got its $240 million and $15 billion valuation from Microsoft (MSFT) last year.

All of this frantic funding activity is, of course, this bubble’s version of going public–grab big cash investments from investment firms and hedge funds, desperate for a good bet on the sector, without the pain of public scrutiny of questionable business prospects that did in Web 1.0 shooting stars.

It’s that or get bought for an ungodly sum by equally desperate Web 1.0 companies (See: AOL+Bebo).

Sources close to RockYou, which has had acquisition feelers put out to it from larger companies in the past, said the company has had several strong offers of funding, but it is trying to select the right partners for the latest round of funding.

“We want our investors to be strategic and helpful to the company,” said one person close to RockYou.

RockYou has so far been funded by Sequoia Capital, Lightspeed Venture Partners and Partech International.

(Interestingly, Sequoia backs another instant messaging and chat widget maker, Meebo, which is reportedly seeking a $250 million valuation, which I posted about here yesterday).

To be fair, makers of highly distributed third-party apps like RockYou are garnering immense traffic and their widgets are syndicated everywhere. RockYou’s Super Wall, which lets you turbocharge your digital wall, for example, is one of the most popular on Facebook.

Other RockYou apps include: X Me, a communications tool that allows you to “Hug Her, Slap Him, Tickle Them!”; and Likeness, where you can “compare yourself with friends and movie stars like Angelina Jolie, Jessica Alba, Keira Knightley and many more.”

The company has been trying to monetize all this traffic and popularity and distribution, as well as knowledge of user behavior, by offering advertisers new forms of engagement.

But the jury is still out on these interesting but unproven efforts by all the social-networking players.

In any case, the money is apparently still flowing into these start-ups, taking a chance on them being the next big media play.

Here are two videos I made when I visited RockYou’s offices in San Mateo, Calif., last October, after I had called the widget market juvenile and faddish.

The first is my tour of the office, where I was playfully accosted by an infant–oops, a RockYou engineer–in a suit. The second is my interview with Shen and Tokuda.

So I recently ventured into the heart of the empire of toddler developers with a visit to the San Mateo, Calif., HQ of RockYou, the super-popular maker of third-party apps on hot social-networking sites like Facebook and MySpace.

I have been on a bit of a grumpy tear of late about the juvenile nature of these widgets, whose use has taken off explosively, as the sites they live on have grown.

I have felt that most of them have been a bit silly, useless and faddish, rather than long-lasting and relevant.

RockYou’s apps, for example, include: Super Wall, with 1.26 million active daily users on Facebook, which allows you to turbocharge your basic posting wall; X Me, a communications app with 706,000 Facebook users, which allows you to “Hug Her, Slap Him, Tickle Them!”; and Likeness, where you can “compare yourself with friends and movie stars like Angelina Jolie, Jessica Alba, Keira Knightley and many more,” which has 611,000 active Facebook users.

Like another widget maker, Slide (I did a post and video on Slide here, as well as a three-part interview with founder Max Levchin too), the start-up has big VC backing. In RockYou’s case, it is funded by Sequoia Capital, Lightspeed Venture Partners and Partech International.

Of course, there are the rumors of big-money buyouts and even IPOs for these developers.

I am not so sure this is a good thing, but I do also believe there is something important going on with companies like RockYou, which could become akin to the major software makers of the past era. If, of course, they grow up a bit first.

Here’s my video of a visit to their office (and here is an accompanying interview with its co-founders Lance Tokuda and Jia Shen), where one employee jokingly played dress-up just like an adult, sporting a suit and tie just for me.

Oh, those crazy kids!