A utility for Linux that attaches to a running process and searches the memory field for strings supplied by the user. Once found, you can then change the string to a different value. This works much like the pokefinder utilities on the 8-bit computers.
FDBG is a code debugger for code running in user mode (ring 3) in long mode (i.e., 64-bit native mode) on AMD CPUs. Ports for Windows and Linux are available. Written entirely in assembly language, includes source code. Extremely small executable.
BinNavi is a tool for performing static and dynamic reverse engineering of executables for a number of platforms - x86, ARM, PowerPC, and MIPS. Can connect to remote debuggers on other systems to examine running code. Extensible with scripts, can annotate call flow graphs. The source is on Github: https://github.com/google/binnavi