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My X61s is the computer in my collection I've owned the longest. It has a 1.83GHz Intel Core 2 Duo processor and 4GB of RAM. I have a 256GB SSD, which is an upgrade from the various other SSDs and HDDs I've used with it. I also own UltraBay accessories that I can use with it including both SATA and IDE drive caddies and multiple DVD burners.
It is my test computer, previously being used for compatibility with my older calculators (the UltraBase x6 has a serial port, which is really handy) or with my SC-880 for backing up patch data and the like. I also used it for MIDI file creation and playback (these duties have since been taken over by AyuPad). Right now it boots Windows 7, although I have put many different operating systems on it. It is an amazing computer that has survived everything I've thrown at it and still keeps working. Its name comes from Tsumugi Kotobuki of the K-On! anime series.
Update 2020-11-01: Installed Ubuntu MATE in a dual boot configuration.
My Windows 7 install (left), my Ubuntu MATE install (centre), and the laptop (right)