The (unfortunately brief) 'Sheldon in Narnia' story.

http://www.sheldoncomics.com/archive/031111.html

On a side note, http://www.sheldoncomics.com/archive/031121.html is fantastic. I love Arthur's expression in the third panel.

Sheldon and Arthur discussing Nostradamus:
http://www.sheldoncomics.com/archive/050505.html

Also, General Zod:
http://www.sheldoncomics.com/archive/060313.html

I have a feeling that's not the first time he appeared, but I don't have time to go looking for it.

In which Arthur molts
http://www.sheldoncomics.com/archive/050603.html

In which Sheldon goes to summer camp

http://www.sheldoncomics.com/archive/050610.html

The Sheldonsoft anniversary party:

http://www.sheldoncomics.com/archive/061127.html

Flaco gets kidnapped (lizardnapped?) by an owl: http://www.sheldoncomics.com/archive/100825.html

Archaeologists find Kryptonite:
http://www.sheldoncomics.com/archive/070426.html

Also, it would be awesome if you could collect the "True life tales from owning a pug"

In which Coco The mime comes to visit and Arthur has a lengthy discussion with him
http://www.sheldoncomics.com/archive/050905.html

in which Arthur tries to eat a spider and starts a personal vendetta http://www.sheldoncomics.com/archive/050819.html

Gramp goes to the doctor: http://www.sheldoncomics.com/archive/051108.html