Last week, I visited Meebo, the Web-based instant messaging company, at their headquarters on Castro Street in Mountain View, Calif.

Why? Well, like a lot of Web 2.0 companies, because it’s a hot and hyped little start-up with a fast-growing audience for its–wait for it!–widgets!

But Meebo makes actually useful widgets, such as its flagship unified instant-messaging offering. Thus, in an endless sea of useless and juvenile apps, that immediately makes BoomTown happy and interested.

And, backed by Sequoia Capital and Draper Fisher Jurvetson, it also has a solid team, although it is one still in search of a much more solid business plan.

Which is: Advertising, of course! (Which is: Web 2.0′s possibly dubious mantra of mantras! Presumably, if you say it enough times, it will come true!)

How hot and hyped Meebo is was in evidence, when this article in VentureBeat posted yesterday claimed that the company was raising $25 million to $30 million at a valuation of $250 million.

Meebo’s last $12.5 million round valued the company at an already kooky $60 million to $70 million. The company was started in 2005.

Ouch, my head hurts from the bubble atmosphere that persists in Silicon Valley, even as our economy is tanking and the Fed can barely prop it up. But I was already in pain at the $850 million that AOL forked over for the very-nice-but-very-not-worth-$850-million Bebo, so please pardon my deep and unfulfilled need for sanity.

Still, it is hard not to like what Meebo is doing, which is a million times more useful than some widget makers. Basically, it solves the interoperability problem in instant messaging, by allowing a user to chat across the most popular sites–AIM, Google (GOOG) Talk, Yahoo (YHOO) Messenger and Microsoft’s (MSFT) chat service.

It also offers chat rooms to sites across the Web, branded as Meebo Rooms, like the one below on Meebo’s valuation, with lots of cool features, such as the ability to play videos that are embedded in them.

These products have attracted tens of millions of unique monthly users, trading over 100 million messages a day, although–as I said–it is not clear how money will be made providing this useful service.

Meebo, like a lot of similar companies, has struck some interesting ad partnerships, including with record companies, but–no matter what anyone says–it is obviously too soon to tell how effectively it will perform.

Luckily, in this video, two of Meebo’s founders–Seth Sternberg and Sandy Jen (the other is Elaine Wherry, who is not in this video)–as well as recently hired CNET vet Martin Green, who is handling business development, explain it all for you!

Here’s the video: