Originally Posted by Hellion
I'm trying to re-get into the series and enjoyed the series/episodes when they delved into the DCU, what season did Smallville delve into the Justice League and DCU?
Season 6; mainly the first half.
They started bringing in and utilizing DC Characters a lot back in Season 5 though, with the likes of Aquaman, Cyborg, Zod, and Brainiac having been introduced into the series for that season, along with the Fortress of Solitude and the Phantom Zone.
If you're mainly interested in the JLA episodes though, then I'd say the first and last episode of season 8 covers the JLA, while the 2 Hour "Absolute Justice" in Season 9 and its season finale covers the JLA and JSA very well. Season 9's "Checkmate" is another good episode as well that deals with JLA material.
The Last JLA themed episode that we saw is probably the one in Season 10 where we see Aquaman and Mera.