Builds:
This section addresses the common, and not so common builds I have seen. Either way, they are the most effective ones I have seen. Note that for all builds, you can use the favorable stat spread to go for feats like dev crit, or in some cases, epic feats like self concealment, or the more easily reached and effective damage reduction.
Druid/shifter/monk-
suggested stats:
str 8
dex 8
con 16+
wis 17, finish at 30+
int 8+
cha 8
The first build posted when HotU came out and for obvious reasons. Wisdom is needed for many of the epic shapes, which in turns lead to very high AC and a bit better stunnning fist DC. The exploitation of the unarmed attack progression allows for 5 attacks per round which without proper building, a shifter has only 3 attacks per round. Not to mention all the other goodies like evasion, improved knockdown, cleave, monkspeed. Unlike other builds, a strong defense, coupled with a strong melee presence, and able to gain a high amount of druid lvls allow for good spellpower. Powerful and versatile, this build doesn't slouch for much of anything.
On the con side, the shifter/monk suffers from being flatfooted. Their reliance on the unarmed forms means a lot of DR mobs make them ineffective. Monk means there's only 3 attacks per round for the weapon forms. And most of the unarmed forms dont merge items, which means the main strength of item versatility is less than other shifters.
Druid/shifter/defender:
Suggest stats:
str 8+
dex 13+
con 16+
int 8+
wis 13+
cha 6
I'm a little biased, I like the defender/shifter a lot. Damage reduction with risen lord or stone golem for example means mobs have to deal twice as much as your DR in items and natural, to even scratch you if doing pierce/slash. (25% for golem pierce though). On top of that, your normally weaker nuker forms like death slaad can always benefit from damage reduction. You can even make most defenders jealous by surviving in spell dangerous areas with rakshasha. Unlike other defenders, who have to spread themselves kind of thin in terms of offense/defense, either losing out on DR by going for dmg reduction, a shifter can potentially have 46 str and 18/- DR as a dragon for example. 4 attacks in any shape means a good melee presence. If you go the low wisdom route by going only 13 wisdom, you can gain dev crit with a kobold commando which has 5d6 sneak attack, 18/- DR, and 26+ str quite easily. Something about seeing, str 28, dex 27, con 21 is oddly satisfying on your character sheet, same for other shapes if you focus on con, the least changed stat, its always nice to have a high hp pool. They also can't get flanked, so as long as you attack, that means no enemy flanking sneak attack. Which means, your normal sneak attack immune form, can now be swapped for a more favorable damaging form.
Cons, gaining lvls in druid takes away from your defense. Relying on shifter for your offense ultimately means you're only as good as another shifter which is not that great. Pre-epic lvling is a pain.
druid/shifter/paladin or blackguard
This build is kind of fun, divine might/shield allows for more damage/AC, and the warrior multi class means 4 attacks per round. I like being dwarf for this build, because your normally ugly dwarf, who has an easy time for epic damage reduction, can now get rid of their ugliness via shifting (i.e. that +2 con is useful for damage reduction, and that -2 cha goes away when shifted). Not to mention, bless weapon, deafening clang, and the stat buffs that a pally has are nice. Try an epic kobold commando with divine damage and acid is hilarious, not to mention 27 dex, and +1 size modifier means your normal whiffing shifter, can now hit with the best of them or run away. This build also has 4 attacks, taunt also helps you hit.
Cons, flatfootedness. Slow pre-epic lvling.
Druid/shifter/wizard:
Puff by cdaullep is the prime example here, a strong spellcasting presence coupled with epic spells, and epic shapes allow for some nice melee and spell options. Not to mention it uses the highest lvl caster class, which is good since you get better SR penetration for rak spells. You can also empower magic missiles, and acid sheathes that no other shifter could. Mage armor helps the shifter a lot more than other classes, and surpasses what monk/shifter could have in terms of wisdom boni.
Cons, very crummy attacks, tends to have to decide between spell power and shifter power.
Druid/shifter/rogue or assassin.
I personally like assassin best, since it stacks with the kobold sneak attacks. The main power of these builds is that you gain offense (in terms of sneaks) and skill selection. The druid/shifter skill list is pretty sad, but put in use magic device, open locks, tumble, etc, and you got a nice picker who gets a boost via shifting, and doesn't die like other pickers nearly as often. Best part is, you can gain this skill selection with a relatively small amount of lvls.
Cons, 3 attacks per round.
Druid/shifter/weaponmaster risen lord
Lockindal posted this first as a viable build, the shifter with a whip/tower shield is pretty nice. Specialized to be sure, but risen lord is not a slouch for any department in terms of immunities, and the bonus to hit covers the weakness of shifters nicely. You also get 4 attacks per round.
The con side is one form, which is a lot, you may need to shift to another form, but you don't get a nice scythe then. To gain that nice attack bonus associated with WM, you need lots of lvls in it, which means the other shapes are hard to gain without great wisdom, and your druid/shifters are low.
Conclusion:
Hope that helps you build your shifter for their 3rd class. NWN is nice since it let's you pick whatever class, and I've seen a lot of builds just go ftr/druid/shifter for example, and not really have much to look forward to and/or use the shifter's strength.