The theme from ABC's wonderful show, Castle, starring Nathan Fillion and Stana Katic. I've looked for this song on YouTube many times and never was able to find it, so I recorded it and am sharing it for everyone else!
If you don't have the mp3 and want it, message me with your email address and I'll send the theme song your way!
In response to common questions, from what I've been able to find out this theme is NOT part of a longer song, and was composed specifically for the show. UPDATE: The show lists Robert Duncan as the composer for the show's theme and score. (Fun fact - he also composed music for the later seasons of Buffy the Vampire Slayer. Nathan Fillion appeared in the final 5 episodes of the show.)