I've tried to find some chess engine source which would be completely free for any use. But most of open source engines is under GPL license. At least LGPL would be nice. Ideally I would like some version which I could change and then use in my non-opensource program. Do you know any?
FYI: LGPL vs. GPL:
The main difference between the GPL and the LGPL is that the latter allows the work to be linked with (in the case of a library, 'used by') a non-(L)GPLed program, regardless of whether it is free software or proprietary software. The non-(L)GPLed program can then be distributed under any terms if it is not a derivative work. If it is a derivative work, then the program's terms must allow for "modification for the customer's own use and reverse engineering for debugging such modifications." Whether a work that uses an LGPL program is a derivative work or not is a legal issue.