Never tried LightTPD, but I've been using LiteSpeed 2.1.18 (free version) for a while, and been very happy with drastic performance increase comparing to Apache. Take a look at this benchmark: http://litespeedtech.com/library/benchmarks/benchmark_r3/ LightSpeed 2.1 Standard (free, but not open source) seems to edge LightTPD slightly for high number of concurence connections.
I tried it briefly, and was pretty impressed I would have stuck with using it if it weren't for cpanel not working correctly with it (my clients need it, not me)