This sounds like a good recurring opportunity:

TopCoder is launching a number of upcoming private bug races in a large, rule-based expert system, and we need more bug racers! The system is written primarily in LISP and C, so if you've had exposure to LISP in school and always wanted an opportunity to improve your knowledge, this is it. We are planning to deliver a steady stream of bug races for the coming months, so joining now would be a good opportunity to get ahead of the rest.

If you're not familiar with bug races, they are short competitions to fix identified bugs in code. You will be presented with a description of a bug, instructions on how to reproduce it, and access to the code to fix it. You'll be asked to implement the fix and commit your changes. More information on the process is in our Wiki here:

http://www.topcoder.com/wiki/display/tc/Bug+Races

These bug races are private, but you can find the public ones here to get an idea of what they typically look like:

http://www.topcoder.com/wiki/display/tc/Active+Bug+Races

The only way to participate in these private races is to express your interest and be granted the necessary permissions. So, if you want to be added to the list, please respond directly.

p.s. If you need a little refresher, Peter Seibel's excellent "Practical Common Lisp" can be read in its entirety here: http://www.gigamonkeys.com/book/

Note: I have no affiliation/endorsement of the above names. Top coder has been a good experience for me and I think it's great that they are offering a chance to work one's Lisp skills--after all, use it or lose it!