Brilliant idea for a thread. Sorry to probably echo - getting all the links has taken a while and I imagine some, or even most, of these orgs will be named already by the time I hit the post button. But you can never have too much of a wonderful thing, right? 
Nia is Karen Ingala Smith's domestic abuse charity. (She heads it - it's not 'hers' - but I mean it has a clear commitment to women only spaces). They're constantly under financial threat for holding the line on single-sex provision. A standing order for just £3 a month will be topped up a bit with your taxes, if you work, and over a year that could amount to around the £40 mark. If a thousand Mumsnetters did that, just think what a difference it could make to women seeking refuge from male violence, in guaranteed single sex provision. Just from three quid a month leaving your account. Most wouldn't even notice it go.
FiLiA is Fawcett for women with self-respect. Bloody brilliant org. Their conference is in Portsmouth in October and it's scaled in cost, depending on your income. Low income/concession rates are just £20 for the weekend (they ask better off women to pay more, to support those who need the concessions). Great way to meet other women concerned to support the rights and needs of women and girls.
The Centre For Women's Justice is a dream. They tackle the law in terms of the rights and interests of women and girls in the courts - in the words of the barrister Julian Norman, "women are great at getting things done - the courts are how we can do that." Fantastic feminist organisation, and very new - only 2016. If anyone can afford even £3 a month or so, over a year that adds up and can make a big difference.
Southall Black Sisters is a human rights organisation for women. They're staunch and dedicated to the rights and needs of BAME women and girls, and have been for more than 40 years. They're staunch as fuck and always were.
Fair Play For Women have a focus on stats, law, and policy around sex and gender. They're clear, lucid, and provide a wealth of evidenced information. Lost count of the number of times I've pulled something interesting from their site, and gone off to read up more.
Woman's Place UK Really great conference, amazing names involved, their work and effectiveness is amply demonstrated by the levels of hate and rage from the misogynist dickwads, and their courage at standing up to be counted at a point in history when hardly anyone would makes them real heroines, in my book. It's organised by Labour women, but the reality is that women all across the political spectrum can find common ground on this. It's our rights - indeed, the very legal definition of what a woman is - under attack, and how you vote doesn't alter that.
Safe Schools Alliance has the slogan, "Putting Safeguarding First" which pretty much sums it up. All kids benefit from safeguarding, but girls especially, for obvious reasons. Anyone thinking teenage girls don't need separate loos from the boys never went to a mixed sex school. One rape reported to the police now for every day of the school year, which will of course just be the tip of the sexual assault and harassment iceberg. Safe Schools Alliance offers a wealth of resources, advice and support for any parents wanting to know WTF to do about the regulatory and institutional capture of our educational establishments. They're supporting the Oxfordshire Trans Toolkit Judicial Review. Really helpful to anyone wanting to talk to their child's school about the batshittery of forcing a secular religion on the whole country, on the quiet.
LGB Alliance also looks after the interests of gay men, but the erasure of same sex sexuality is primarily aimed at women, for reasons that should be obvious. LGB are not anti trans, contrary to the enraged claims. Trans people need, and have, their own orgs because their interests are not the same as LGB people's. It's therefore a bit of a puzzler that there's so much rage that they also want an org that puts their own interests ahead, in advocacy. As with women, apparently the T need their own specific groups, but nobody else may.
Standing For Women is Posie Parker's org. I know she's divisive, but anyone arguing that we'd be where we are right now without her is, I think, deluding themselves. Yeah, she'll talk to anyone who'll platform her. The left wouldn't platform gender critical views because of #NoDebate, so she had to go to those who would. The rage that she wouldn't just shut up, sit down, be a good little girl and do as she was fucking well told is nakedly misogynist. Love her or loathe her, she's Standing For Women, and I remain grateful.
If there are groups now for professional women (academics, doctors, teachers, civil servants - gulp - for example?) and someone can link them, that would be aces. If not, I hope they are started soon. There's power in numbers. They can't fire everyone. As Labour are, somewhat to their horror, discovering - there are way more of us than there are them.