This first one is of a Tau Commander, the art coming from the original Codex Tau (I believe the same picture is in the current Codex: Tau Empire as well. It required a minimum of converting--I just had to trim off the lower legs at the knee joint and reattach them to make them bend more.
I always thought that this picture of Commander Farsight looked pretty cool, so I felt inspired to duplicate it. This one also required a minimum of conversion, I think it was just bending the knee a bit like with the Tau Commander.
Next is an assault sergeant from the new Blood Angels Codex--I thought that the Blood Angels marine with the pair of Lightning Claws looked particularly cool. This one required some more converting. I used one Lightning Claw from the Space Wolves Grey Hunters set (generously donated by Badelaire), while the other is from the Space Marine Commander's box set. I had to chop them off at the elbows and rotate them to get the angles right. To match the breastplate, I used a normal tactical marine breastplate and filed off the skull, replacing it with a blood drop from the Death Company sprue. The pauldrons (er...shoulder pads) are from the Death Company sprue and the Sanguinary Guard sprues. For the legs I hacked off the right leg at the hip, and replaced it with another assault marine right leg (so now both legs are the ones from the assault marine sprue that are more bent at the knee). I'm still working on the conversion (I have to greenstuff some power cables), so this is still a work in progress.
This last one is a model that Badelaire did, based on the picture of a particularly mean looking nob in Codex Orks. The arms and lower legs are from the Warhammer Fantasy Black Orcs sprue, while the choppa is converted using normal Ork Boyz bitz. The rest of the bitz are standard ork nobz bits.