Seesmic Desktop 2 has just arrived, and it's bringing an entire marketplace of apps along with it.

The new platform will give its users all-in-one access to some of the most popular (and some of the more long-tail) social media services around. It includes the usual suspects, such as Twitter and Facebook, but its plugin marketplace also includes services such as Zappos, Formspring, YouTube, Last.fm, Techmeme, Ping.fm and many more — around 40 apps and services, all told, with dozens more coming soon.

Users will be able to add, remove and customize what services they use via SD2, leaving only the apps that truly matter to them. With broad customization options and tons of flexibility, Seesmic Desktop 2 seems poised to be all things to all users, a sort of social media meta-app. It's a beautifully built UI with subtle, elegant details.

SD2's initial lineup of integrated apps includes Facebook, Foursquare, Twitter, LinkedIn, Google Buzz and Ping.fm. You can expand SD2's functionality throught the Plugin Marketplace. There are too many apps available in the marketplace to list here; you'll find all the services we mentioned above, plus some longer-tail services that will appeal to various subsets of Seesmic's user base, including Bing Maps, Klout, Salesforce, OneRiot, Ning and Socialcast.

Of course, the app will retain the familiar interface of customizable columns for sorting and seeing all your Facebook or Twitter updates, mentions and messages; but it now has the capability to do a lot more as well. Here are some screenshots showing just a few of the nifty things SD2 can do:

And if there's a service you love and use but don't see — or if you're a developer and want to get in on the SD2 platform — there's an SDK for that. You can choose to build a plugin yourself, or you can contact the Seesmic team directly to help them with developing a plugin for your app of choice.

SD2 is a Silverlight application and is Windows- and Mac-compatible. Linux users, we hope you like using emulators; until Silverlight and Liinux distros can play nicely together, SD2 won't work so well with your OSes.

Back at Twitter's developer conference, Chirp, Seesmic founder Loic Le Meur shouted out over a crowd of shaken third-party Twitter developers, "There is plenty of opportunity for us." In a time of uncertainty for many third-party services, Seesmic has certainly made the most of the many available opportunities by adding a maximum amount of value and integrating an unprecedented number of social services.

Looking back at our interview with Le Meur during Chirp, we see how his words foreshadowed today's announcements. At the time, he told us that innovative applications that add value and diversity will always find a place in the market; adding value and diversity is precisely what Seesmic has done with SD2.

Give SD2 a whirl for yourself, and let us know what you think in the comments. Is the app convertible enough to meet all your needs? What services do you think SD2 should work on adding next?