My character is Bard/Marshal that just hit level 15 (10brd/5mar). I have already decided to level up the bard class as that will allow access to 4th level spells. I have completely and totally embraced the support role (most of my spells are of the "buff an individual or the party" or "re-roll dice" variety, I have a weapon and have never used it and the only offensive ability I have and use is the Doomspeak feat). The question is what feat to take next.
I have the following:
Combat Expertise
Skill Focus (Diplomacy) [bard class feature]
Extra Music
Extra Music
Subsonics
Lyric Spell
Doomspeak
With my CHA as high as it is I already get 18 bardic music uses per day. But another Extra Music is not out of the question (last two sessions (representing one day of adventuring) the party had no chance to rest and so I actually used all of my bardic music uses in that one day, but that is an exception; all previous sessions, since I created the character, I never got close to using all my bardic music)
I thought about the Dimensional Jaunt (Reserve) feat. Since I just got access to 4th level spells I can learn dimension door and be able to use the DJ feat. My reasoning behind this is that it would give me a way to get out of harm's way (only having 87HP and AC27 (with combat expertise) makes me squishy)
I thought about the Obtain Familiar feat but the risks outweigh the benefits.
Any feat from any official published source is acceptable.
You're missing pretty much all of the Inspire Courage support feats - Song of the Heart (EbCS), Words of Creation (BoED), Dragonfire Inspiration (Dragon Magic - this gets much deadlier if you've got a sonic-type dragon heritage (Battle Dragon is a perfect thematic fit) and combine it with the Creaking Cacophony (Spell Compendium) spell), and so on. Of these, Words of Creation easily has the biggest benefit as a single feat, but also has the highest ability score requirements. Song of the Heart is the least intrusive, if you worry about overshadowing.
Don't worry about Dimensional Jaunt. Handle your defenses with equipment. I'd specifically suggest a Shadow Cloak (Drow of the Underdark) and Tome of Battle equipment (the Iron Heart Vest linked to Wall of Blades is a great last-ditch defense, arguably much better than raising your AC and certainly more cost-effective with your buffs).
Also, buy a Vest of Legends (DMG2) and Badge of Valor (MIC - the rest of the set isn't bad either if you've committed to using marshal) if you haven't already, and look into wands of Inspirational Boost (SpC).
Thank you for all the advice. Words of creation or song of the heart are great options I did not even know about 
(very hard to dig through the nearly available 1000 feats).
DaBeerds wrote:
Beyond Words of Creation, and Song of the Heart - you also should be using equipment to completely push that optimization to the limit:
Badge of Valor (Magic Item Compendium)
Masterwork Instruments:
Drums: +1 to the bonus to damage w/ Inspire Courage, but -1 to the bonus vs charm and fear.
Fiddle: +1 to the bonus vs charm and fear w/ Inspire Courage. If you have 5 or more ranks in Perform (Dance) the bonus increases to +2.
Flute: +2 to countersong checks)
Harp: +1 target additional creature with Fascinate and Inspire Greatness
Horn: +1 to the bonus to damage and save vs fear w/ Inspire Courage, but the effect ends 1 round after the recipient can no longer hear the bard toot.
Lute: +1 level to determining effectiveness of bardic music.
Mandolin: +1 to the attack roll bonus w/ Inspire Courage, but -1 to everything else (damage, save vs charm/fear).
Read more: http://www.enworld.org/forum/showthread.php?185372-Inspire-Courage-Optimization&s=40fdca1e3504ca1a1504106dd4984a52#ixzz2kqzu5qWG
Mandolin is really good with Dragonfire Inspiration. A Lute will combine well with a Vest of Legends to always put you one full step ahead with Inspire Courage. When you get to Inspire Greatness the Harp's pretty nice I must say.
I've always played bards that keep both hands busy, so no instruments for me, but the bonu's are quite good.