So, after reading up on all the other Factions of Sigil in preparation for a game of Planescape, you might have found yourself thinking; "Wow! These guys are all nothing but perverts, losers, weirdoes, psychos, assholes and/or freaks! I don't want anything to do with these jerks!"
If that's the case, then the Free League is for you.
See, not everybody in Sigil buys that crap the Factions are slinging about belief defining reality and the importance of needing to discern the true meaning of it all. Ask ten of these rebels why they don't, and you'll get at least ten different answers for it. But the Factions didn't get to be the powerhouses they are in Sigil without being willing to get pushy about what they think is right.
So, the Indeps - short for "Independent Souls" - reluctantly united under the loose banner of the Free League, which is essentially "the Faction that isn't a Faction", united only by a common cause of personal freedom, a willingness to respect others in pursuing their own independent quest for discerning meaning in their lives, and an oath to band together and kick seven shades of shit out of any puffed up Faction-flagbearer who wants to rough up some non-believers.
Unfortunately, this isn't as effective as it could be. The vaunted independence of the Free League means they have no real organization to them. They have no leaders to unite them and help them coordinate, no Factol to give them protection in the legal arena. Standing up for yourself and relying on your own strength is a noble belief - but the truth of the matter is that having friends at your back is something you need, sometimes.
And the Free League needs it. They have the less than desirable "honor" of being the most hated group of people in all of Sigil. Other factions generally look down on them for not having the guts to take a stand (preferably with them) and believe in something greater, with only the Xaositects, the Fated and the Bleak Cabal having anything like respect for them, but the Harmonium truly hates them. After all, the Indeps' belief in independence, free-thinking and being your own person is anathema to the Hardheads' fixation on group-think and tyranny "for the greater good."
The end result of this is that, at the time of the publishing of The Factol's Manifesto, a secret collection of evil-aligned Hardheads is illegally arresting, imprisoning and outright murdering any Indep they can get their hands on. True, it's not a conspiracy involving the three legal Factions like the Free Leaguers fear, but that doesn't change the mechanics. Nor does it change the fact that if Sarin, the fucking Paladin who runs the Harmonium, will essentially take no action to punish them and will only scold them for it because they have yet to receive orders to do so, not because, y'know, that shit is evil and he's supposed to be Lawful FUCKING Good! No wonder the Indeps hate the Harmonium right back, am I right?
Ironically, the plague is kind of their fault. See, when we said that everyone hates the Indeps? We mean everyone. Including Her Bladed Serenity; the Lady of Pain herself. Back during the Great Upheaval, the Free League's ranks swelled as thousands of people who either didn't like the new Factions that were taking power or just never liked the Factions at all flocked to the one group who said "think for yourselves", and at their peak they numbered over a million registered members. Now, the Lady of Pain didn't like that at all; the other Factions she could easily manipulate, but a band of a million neutrally-aligned, chaos-tinged free thinkers? There was no carrot she could dangle in front of that if she felt it necessary. So, she started to cull the ranks, Mazing and shadow-flaying huge numbers of Indeps until the Faction dropped to a more reasonable size. Some 600 years later, the Indeps choose to blame the mass deaths on a plague -- and, out of a perverse sense of humor, the Lady of Pain has obliged them by releasing a real plague that does only target Indeps into the city.
Needless to say, the dissolution of the Factions after the Faction War is a huge relief to the Indeps; after all, they never were a Faction, and now they're the norm and not an outlier. They just went back to their daily lives, enjoying the freedom from Harmonium bullyboys and goongirls.
Joining the Free League is easy enough; find a member and ask to join. If you prove you got the right mentality (not strongly believing in any one belief and prizing independence), and aren't scared off when they warn you that you get no political power and few rights as an Indep, then you're in. Technically, it's open to any Alignment, but this Faction's emphasis on independent thought, freedom and independence resonates most strongly with Neutrality and Chaos and the few Good and Evil members who join are distrusted, which means they don't get access to the Indep Underground, which is one of the few benefits of this Faction - after all, they don't have a lot of power.
Membership does provide a +2 bonus to all saves against mind-altering effects, and can even resist normally perfect attempts to alter their minds (though without the saving throw bonus). Also, Indeps stick together, so in a public brawl, a member of the Free League has twice the chance of receiving spontaneous help from bystanders if they're also Indeps, and an Indep gets a 20% discount on items from the Great Bazaar, which the Indeps run. Finally, after hitting third level, an Indep is trusted enough to be clued into the Indep Underground, doubling their chances of picking up chant when they go scouting for info.