Overlord
Overlord is a build system written in Common Lisp. It is a real build system, with the features you would expect of any reasonably modern build system: rules with multiple outputs, parallel builds, immunity to clock issues, and dynamic dependencies. It also solves certain problems specific to Lisp programming (namely, it lets you you reproducibly specify the desired state of a Lisp system which is to be saved as an image.)
For more discussion of the thinking behind Overlord and how it relates to other build systems, consult the wiki.
Advice for users
Before using Overlord, it would be a good idea to make sure you are running the latest version of ASDF.
Note that, to run the test suite, you will need to download Core Lisp, and, if not on Windows, you must have the touch program in your search path. (On Windows, Powershell is used instead).
Overlord stores its persistent data structures in a cache directory. On Linux, this is $XDG_CACHE_HOME/overlord. The data structures stored there are versioned. It might worth checking the cache directory from time to time to delete obsolete files.
Overlord is developed and tested on Clozure and SBCL. In the future it may officially support other Lisp implementations, but that is not a priority.
Examples
Here are some projects that make direct use of Overlord:
cl-https-everywhere. In-process HTTPS Everywhere rulesets, automatically fetched from the HTTPS Everywhere repository and compiled into Lisp code.
Proctor. Proctor treats tests as build targets, allowing you to precisely specify their dependencies and re-run tests only when necessary.
Vernacular. Provides a module system for embedding languages, with arbitrary syntaxes, into Common Lisp systems.