Here's a simple one that I actually got a late start on last year, and finished sometime in January.
BeforeWell, I forgot to take a picture before I started, but here's the shirt as it was pictured when I bought it on eBay:
It was a really simple T-shirt with a small graphic of a "Scary Cat" on the front. I loved it because I love cats, but it didn't fit well at all, being too short and too wide.
The fix was pretty basic—I unstitched the bottom hem, took in the sides a little bit, and added a small strip of black fabric to the bottom, then, because the black strip looked kind of weird all by itself, I widened the neckline and gave it a black binding as well.
I really think the addition of the black trim not only made the shirt fit better, but it also made the cat look better, being surrounded by similar colors, instead of all alone on a field of orange.
More pictures and more details about the process (though not many) on my blog.