This article is from the Frequently Asked Questions for Linux, the Free/Open Source UNIX-like operating system kernel that runs on many modern computer systems. Maintained by David C. Merrill with numerous contributions by others. (v1.0).
A: ld.so is the dynamic library loader. Each binary using shared libraries used to have about 3K of start-up code to find and load the shared libraries. Now that code has been put in a special shared library, /lib/ld.so, where all binaries can look for it, so that it wastes less disk space, and can be upgraded more easily.
ld.so can be obtained from http://tsx-11.mit.edu/pub/linux/packages/GCC/ and mirror sites. The latest version at the time of writing is ld.so.1.9.5.tar.gz.
/lib/ld-linux.so.1 is the same thing for ELF ("What's all this about ELF? ") and comes in the same package as the a.out loader.