The War of the Ninepenny Kings (alternatively known as the Fifth Blackfyre Rebellion), was a rebellion in the Free Cities which threatened to cross the Narrow Sea via the Stepstones and invade the Seven Kingdoms. It was fought almost 25 years before Robert's Rebellion. However, the invasion was preemptively crushed in an assault on the Stepstones. Ser Brynden Tully and Ser Barristan Selmy are famous for their heroism during the war.
In the books
In the A Song of Ice and Fire novels, the "War of the Ninepenny Kings" was the last of five rebellions by House Blackfyre, a cadet branch of House Targaryen, which attempted to take the Iron Throne. While the previous rebellions had been fought in the Seven Kingdoms, after which the Blackfyres would retreat back to the Free Cities, this time around they came up with the new strategy of establishing a sizable powerbase in the Free Cities first, and then mounting an invasion of Westeros, hoping that noble Houses disaffected with Targaryen rule would join them in rebellion once they landed.
The last member of House Blackfyre living at the time was a grandson of Daemon Blackfyre, Maelys Blackfyre. He was popularly called "Maelys the Monstrous" because he actually had a small second head sprouting from his neck, the remnant of a parasitic twin that he absorbed in the womb. Maelys was also captain of the Golden Company, a sellsword company founded by the Blackfyres to try to maintain a standing army even when they had no support for a new rebellion in Westros. Maelys combined with a group of merchants, pirates, mercenaries, and other assorted ne'er-do-wells to form the "Band of Nine", who intended to carve out kingdoms for themselves in the Free Cities and Westeros.
Their first act was to carve out territory for themselves in the Disputed Lands between Lys and Myr, they sacked Tyrosh and set up one of their members as the new Archon. They then captured several of the Stepstones, as a base to launch their invasion of Westros from. However, the Targaryens responded before the invasion could commence and invaded the Stepstones to root them out. Brynden Tully and Barristan Selmy performed exceptionally in the fighting, and Ser Barristan personally killed Maelys the Monstrous in single combat, ending the rebellion and the line of House Blackfyre. It would be another six years before the Band of Nine's holdings in Essos were completely dissolved by local rebellions.
Ser Barristan was made a member of the Kingsguard for his deeds (some years later as soon as a new position opened up), and Brynden Tully gained almost equal fame for his military and strategic leadership: the young Jaime Lannister was eager to hear of his heroic exploits. Another result was that during the war the Tullys became associated with the father of Petyr Baelish, and agreed to raise him as a ward at Riverrun. The unforeseen consequence of this was that the young Petyr became infatuated with Catelyn Tully during his years at Riverrun.