Lua

Description

Lua is a very nice and easy-to-learn scripting language specifically designed to be embeddable and able to represent complex data.

Clean syntax
Simple, yet extremely powerful and extensible, data structure: tables (can be used as hashtables, vectors, arrays, lists, namespaces, objects, ...)
Easily embeddable into existing projects
Lightweight
Very fast
Compiled to bytecode, which can be saved to a file
Easy integrated with all languages that can load C libraries (also FreePascal
Many other nifty things ;)

Lua is probably the most popular scripting language for roguelikes. Some of the projects that use Lua:

Angband - between Angband 3.0.0 and 3.0.6. Since then it no longer supports Lua scripts.
DoomRL - internally keeping all it's data in Lua. Since 0.9.9.1 a limited form of modding using Lua is possible.
Portralis - uses Lua a lot to create monster attacks and complex story and quest events. Many gameplay mechanics are now entirely script-based, and future versions will aim to expand it's use even further.
ToME - a good part of the engine is in Lua and the game itself will be completly in Lua in the next version.
H-World - all modding in this engine is done through Lua scripts
Necropolis (Timothy Pruett) Using Lua scripting and custom-built tools users can create new areas, classes, items, etc.
Bone to be Wild Runs with T-Engine from ToME.
ODE Runs with T-Engine from ToME.
Broken Bottle made using the T-Engine4
Run from the Shadow made using the T-Engine4
Harrowed made using the T-Engine4
Ascii Wilderness made entirely using LUA 5 and the Slang graphics interpereter.