ezSIFT
An easy-to-use standalone SIFT library written in C/C++
...The ezSIFT library provides a standalone and lightweight SIFT implementation written in C/C++. The ezSIFT is self-contained, and does not require any other libraries. So it is easy to use and modify. Besides, the implementation of the ezSIFT is straightforward and easy to read.
Wiki page:
https://sourceforge.net/p/ezsift/wiki/