Yahoo plans on announcing Thursday that it has bought Xoopit for a price in the $20 million range, according to several sources, one of its first acquisitions in a long while.

Reached late this afternoon by BoomTown, a Yahoo (YHOO) spokeswoman declined to comment about the acquisition. Xoopit did not respond to emails earlier today.

But sources said it was a done deal to buy the San Francisco-based social email start-up that finds photos, video, links and other files in email so that users can surface and then share them on many sites. It also has other products that essentially enliven email.

Xoopit’s investors–Accel Partners and Foundation Capital, along with several angel investors–have pumped about $6.5 million into the company since 2006.

According to sources, Yahoo was first impressed with its innovative plug-in that works with Gmail from Google (GOOG), and has been looking at the company for a while, previously offering about $10 million for it.

Xoopit also makes a similar photo-sharing application for Yahoo! Mail, which it launched late last year.

The opening up of its popular email product to a variety of third-party applications, in order to make it more robust, has been a goal of Yahoo recently, as it seeks to socialize one of its most popular products.

One source said Xoopit was a good fit for Yahoo because it allowed the company’s email to be the platform that could knit together other social networking services, such as Facebook. This is seen as a core feature, although the Silicon Valley-based company is also opening its email products up to outside developers.

Yahoo has been planning on announcing the acquisition on Thursday at Fortune magazine’s Brainstorm Tech conference, where SVP of Applications Products Bryan Lamkin is appearing on a panel titled, “Dollars & Demographics: Capitalizing on Demographic Trends.”

Lamkin, a former Adobe Systems (ADBE) exec, now runs the Yahoo unit that includes email and other communications and communities products. He was hired in April by CEO Carol Bartz.

That panel topic is a little ironic, several sources joked, since Xoopit is not profitable and has very small revenues thus far.

The Wall Street Journal also reported on the deal, although it said Yahoo and Xoopit were still in late-stage talks and the deal was not complete yet.

But, according to my sources, it is done.

Internet navigation browser plug-in maker Cooliris, which lets users “see” media on the Web in a more visual manner, has gotten $15.5 million in additional funding from its investors.

The new round includes Kleiner Perkins, Cooliris’s initial investor, as well as DAG Ventures, the Westly Group and T-Mobile’s T-Ventures.

Cooliris had already received $3 million two years ago from Kleiner.

The Palo Alto, Calif.-based start-up is also releasing the latest upgrade to its 3-D “wall” software out of beta, which adds more online content from around the Web, including from social-networking sites like Facebook and from a user’s own hard drive.

The trend toward making the Web browser look like a television screen and navigating information more visually is increasing all over the Internet, from video services like Boxee to visually-oriented search services like Searchme.

“We’re trying to allow users to discover, navigate and consume more of the Web in the same amount of time, using visual means, than people do now through text links,” said Austin Shoemaker, Cooliris co-founder and CTO in an interview last week with BoomTown.

Launched last year with a plug-in called PicLens, Cooliris said that its software had been downloaded 10 million times and that it has three million active daily users. It also has a popular iPhone version of its software.

Here are two more Cooliris screenshots like the one above (click on any of them to make them larger):

While rumors of its impending re-sale have apparently been greatly exaggerated, what’s true about StumbleUpon is that its new Web-centric look and feel and a new partnering program represent a major shift for the online discovery service.

The San Francisco-based company, which was founded in 2001 and sold to eBay last year for $75 million, is announcing tonight that users will no longer have to register or download its toolbar to “stumble” the Web.

Users can now simply start on StumbleUpon’s site, for example, and stumble all over the Web using their Web browser as guide rather than a toolbar.

The move is being made simply because most Internet users are increasingly loath to install Web plug-ins like toolbars, a requirement that naturally has slowed the growth of StumbleUpon’s service over time.

Currently, StumbleUpon has about six million registered users, although only a fraction of those are responsible for the approximately 12 million daily “stumbles,” all using a toolbar.

“We wanted to attract users who do not want to use a toolbar, making it easy so they could use the service right from the get-go,” said Garrett Camp, co-founder of StumbleUpon, in an interview with BoomTown earlier today.

Camp noted that that the toolbar–which has been downloaded between 11 and 12 million times–has seen that growth slow over time. Nonetheless, it is not being eliminated either.

“[Toolbar adoption] was still growing, but not accelerating,” said Camp. “Being able to stumble without one was the biggest feedback we got from users.”

Along with the Web-stumble change, StumbleUpon is also unveiling a redesigned homepage–see an example of it below; click on the image to make it larger–which is an attempt to make it more of a destination.

With the new look, visitors can find content by topic and more related to interests. Other changes include a new look for profile pages, as well as user reviews, rating and comments.

Along with its distribution shift and site renovation, StumbleUpon is unveiling a partner program called StumbleThru that will allow visitors to discover content within those sites without going to StumbleUpon.

Sites–starting with HowStuffWorks.com and the HuffingtonPost.com and followed within weeks by RollingStone.com and National Geographic–will display a StumbleUpon “badge” or custom widget.

It is not unlike similar buttons that now dot Web pages from news discovery services like Digg, which users can click to find related pages.

Essentially, much as Google (GOOG) delivers custom search within Web sites, StumbleUpon is offering custom surfing, giving publishers StumbleUpon technology to allow its users to surface content within their sites that is often deeply buried.

As to the blog reports that eBay (EBAY) had put StumbleUpon up for sale after owning it for a little more than a year, Camp essentially dismissed them, noting that the unit is still operating as an independent subsidiary of the auction giant.

“They have given us a lot of runway,” said Camp.

Here is the new front page of StumbleUpon:

Also, here is a video I did last year at the exceptionally noisy (sorry!) party that StumbleUpon threw after it was sold to eBay a little more than a year ago: