Which interestingly seems to be not as easy as it seems. I remember some discussions on how to waterproof incompatibility with the GPL (for a BSD license) and while it initially seemed just having advertising clause would be enough it later turned out even that wouldn't be a 100% guaranty. I don't think a final solution was reached either. Lacking a better option i'd probably still recommend a BSD license including the advertising clause (and have used it myself in the past) but any better recommendation would be very welcome. I write mostly BSD code and i don't want it to become GPL'd just because some clown added 3 lines or linked it to some other project...