This is pretty cool - Spotify's new "Spotify Play Button" has launched, allowing content creators to embed Spotify songs and playlists directly into their blog. This includes regular blogs (like this one), or other services, such as Tumblr. If you've done any kind of embedding before, it's a pretty simple process.
1. Open Spotify and choose your song or playlist. Right click on it and choose "Copy Spotify URL"
2. Go to this Spotify developer page and paste in the URL.
3. Click the "Copy Embed Code" and then paste it into your blog software.
As you can see here, I've embedded one of my favorite songs, Funky Town. Enjoy the song!
As more people learn about this functionality, you can expect to see many more songs and playlists being embedded in blog postings.
The folks at Mashable have posted a more detailed "How to" video if you want to learn more about the new button.