Seeker of the Song and Sublime Chord from the Complete Arcane book are both solid PrC's. The first lets you use your bardic music abilities to do some neat things. Each of the songs they get have two functions, the first is usually some resistance to an element and in the second round you turn the song into an attack as a swift action. You also get an ability to start two songs as a standard action. The big downside is that you focus on music so much that you no longer gain new spells.
Sublime Chord is the opposite. You gain spell levels faster becoming more like a sorcerer with access up to 9th level spells. You get 3 or 4 of new bardic music abilities.
I went with seeker of the song and took my bard to 18th. With the extra music and lingering song feats he's a lot more handy in combat. Coupled with the 2-songs-at-once ability I can get the buffs out faster and switch to damage dealing faster.