Last by Dante has 3 keys solos.

If a song ever needed to be brought OUT of limbo, this is it.

Nouakchott by Matthew Rybicki

-“Discord (EuroChaos Mix)” will join its remixed version in the Rock Band Network, bringing with it...a dubstep bass/drums dual solo. That’s right. You heard me. A DUBSTEP BASS/DRUMS DUAL SOLO.
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The Distance by Drugstore Fanatics has a short keys solo at 2:28.

Be With Me by Goin' Places has a pretty nifty solo.

EDIT: And "Sticky" by Yogurt with Sprinkles.

Christian Muenzer-Victory (@2:55) (being authored)

EDIT: Also, "Top" by Look Left has a nifty sax solo. (released)

EDIT AGAIN: Now, In the Wake of Evolution by Kaipa is getting in on the fun.
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So does From the Blue/Point of No Return by An Endless Sporadic (go to 3:47).

EDIT: And Re-Live by Strikken (being authored) (go to 2:53 to see all hell break loose).

Impulse got pulled due to issues with the keys part (and I also heard that the practice sections were screwed up).

Masterchef Synesthesia (Buttery Biscuit Bass) - Swede Mason

All My Friends Are Herbivores - Ultra Saturday

EDIT:

Lake - A Talking Fish

Heroism - Mystakin

...And also "Miracle", by Silent House.

Also, last I saw, Homestuck's "Sunslammer" had a drum solo marked during the breakdown. However, it's only kick and snare, and does not feel solo-y, so I expect that the authors will make the solo...

Ascension's "Orb of the Moons" for the Being Authored section.

Hagroth - Prey of the Dragon (Laszlolåten)

All hell breaks loose at the seven-and-a-half minute mark.

Gigakoops' "The Final Boss" has a seriously shredding synth solo on bass. Now if only bass started with S.

Also, TWO bass solos in Minotaur Project's "77345_018".

Two bass solos sighted in Minotaur Project's "77345_018"!

Do the Donkey Kong

And Gigakoops' "The Final Boss".

Gigakoops will have a crazy drum solo in "The Final Boss".

No, Everyone I Know...does not have Pro drums. Too early.