I've noticed there are a lot of great job combos that can work EXTREMELY well, but not everyone knows about them. So, I thought I'd throw the few I've seen out there for everyone to look over. My hope is to get the word out that these are very valid combos, so maybe people will start mixing it up a bit and we'll see a little more variety. These are the ones I've seen that worked surprisingly well.
DRG/WHM - Yep, there's already a topic on the board about this. Yup, it's what prompted me to make this topic. It's really a brilliant combo that just gets better the higher you go. I'd really suggest going DRG/WAR until at least 50 though, since through the early and middle parts of the game a DRG/WAR can outdamage almost every other melee. But once the DRG's utility starts dropping off, DRG/WHM becomes another alternative. Cure IV for 7 MP...? Who can pass that up?!? Plus there's Cure Breath which removes any status effect with a -na spell (Paralyze, Poison, Silence, Petrification, Blind, Curse, etc.)
THF/RNG - This one seems to be catching on a bit, but still seems pretty rare. Equipping a RNG sub will allow the THF to use Wide Scan for pulling, and also gives them access to the ranged weaponskills. And give this guy a crossbow and some status bolts and he becomes seriously awesome. Bloody Bolts almost always drain HP from the mob and give it to the THF. Acid Bolts almost always severely weaken the mob's defense. A THF/RNG can be an extremely effective debuffer/puller, and builds TP quite quickly for SA/TA. The ranged weaponskills are a nice bonus that you probably won't use very often, but they tend to chain well so they can be occasionally useful. But really, the big bonuses are Wide Scan and Sharpshot, which make you a more effective puller and give you a lot more usefulness between SA+TAs, making THF no longer the "one-minute wonder". Additionally, a THF/RNG equipped with a crossbow and Bloody Bolts can actually solo pretty damn well too!
SAM/RNG - Similar to THF/RNG, but even better. SAMs naturally are very skilled in bows (second only to RNG itself) and this combo allows them to access those powerful bow weaponskills. With their rapid TP buildup, they can actually do quite nicely as SAM/RNG. It also expands their skill-chainability quite a bit to have access to the ranged skills as well as great katana. They lose their (already marginal) ability to tank, but it's no real loss.
THF/SAM - A pretty good combo, since it boosts the THF's main role in the group. A THF's biggest strengths lie in SA+TA. Obviously you want the THF to be pumping as much hate as possible into the tank. The best way to do this is to combine SA+TA with Viperbite. Store TP makes the THF build TP even faster, meaning more frequent Viperbites. Meditate means a great big boost to TP whenever they need it. Third Eye is great before a pull. Resist Blind doesn't kick in that often, but the last thing you want is for your THF to be blinded so it's definitely a bonus. Without Dual Wield the THF's DEX is a little lower, but the incredibly fast TP gain offsets this.
DRK/WHM - A lower-level combo, but probably the best for DRK until 30 when they can learn Berserk from /WAR. Before that, WAR doesn't really give them anything except Provoke (and DRKs really shouldn't be trying to get even MORE hate). Since DRKs are a hybrid spellcaster, they already have base MP to draw from. So a DRK/WHM is going to have significantly more MP than a WAR/WHM ever would. And since no other sub gives them a whole lot until 30, it's definitely a help being able to throw out Cures as needed. I've actually seen a DRK/WHM be an effective main healer (with a backup healer) in the dunes. I wouldn't recommend trying to main-heal, but it CAN be done. At any rate, this combo loses its utility at 30 but until then I can't think of any reason NOT to use it. HP isn't a real concern until Soul Eater, the STR difference between /WAR and /WHM at that level is minimal, and it makes the DRK a lot more useful early on when they're usually still pretty weak.
So, those are the "unusual" combos that I've seen played very effectively. Most of them are situational...sometimes the "traditional" combos would be more effective (depending on your group/mobs etc.) but often these can be just as good or even better. And it's nice to know you're not THF/NIN #8011936 on your server. ^_^