Good prestige classes for a barbarian:
-Tribal Protector (if you're a half-orc or your DM eases the racial requirement).
-Devoted Defender
-Tempest (if you go the two weapon fighting route)
-Templar (if that's what you want to play)
-Holy Liberator
I'm sure there were others but those are the ones I could think of.
Don't discount a high-level core classes only barbarian either.
-You'll get more rages so you won't need to guess which fights justify raging anymore.
-You'll get greater rage
-You'll get DR (I'm not sure how good it really is--I only had one character who ever had this (Pal 3/Clr 3/Templar 3) and I forgot about it when I was playing him).
-You'll get Uncanny Dodge (Can't be flanked)--never underestimate this against rogues.
You can still multi-class with fighter, cleric, ranger, and rogue very effectively.
