The old Catachan codex has interesting stuff and units like the single elite snipers that pop up in jungle terrain and shoot their special toxin ammunition - wounds on a rerollable (!) 4+. And you can take them out of the game whenever you want so they don't get killed (I think they don't count as kill points for the opponent then!).
Furthermore, booby traps are fun, too. There can be great tactics in placing a plasma charge here and a spike trap there and whoops! there goes a scoring unit and your main tank!
I never played it, but I just ordered some catachans for my
IG mechvet army. I might give the old codex a try if my opponents let me.