Since nobody's made a GP2/GP3 or similar thread, I guess I'll start one. There are a very large number of junior series (far too many in my opinion), so I won't be able to cover them all in detail. Nonetheless, I hope this sparks some discussion and saves interesting news from being buried in the off-topic thread.
FIA sanctioned
European Formula 3 - http://www.fiaf3europe.com
GP3 Series - http://www.gp3series.com
GP2 Series - http://www.gp2series.com
Renault championships
Formula Renault 3.5, Eurocup Formula Renault 2.0 - http://www.worldseriesbyrenault.fr/en/Pages/Default.aspx
Formula Renault 2.0 Alps - http://www.renaultsportitalia.it/campionato/f-renault-2-0-alps
Formula Renault 2.0 NEC - http://necup.com
Formula Renault 1.6 NEC/Nordic - http://frcup.com , www.fia-nez.eu
National championships
Formula Renault UK - http://www.renaultsport.co.uk/championships/formularenaultbarc
Formula Ford UK - http://www.britishformulaford.co.uk
British Formula 4 - http://www.formula4.com
Formula ADAC (Germany) - http://www.adac-formel-masters.de
French Formula 4 - http://www.autosportacademy.com/gb/competition
Formula Abarth (Italy) - http://www.acisportitalia.it/ABARTH/
Euroformula Open (formerly Spanish Formula 3) - http://www.f3open.net
British Formula 3 - http://www.formula3.co
German Formula 3 - http://www.formel3.de/en/
Making things more confusing and adding to the level of redundancy, Renault plans to set up additional 1.6 litre series in the near future. (http://www.paddockscout.com/formula-renault-1-6-alps-series-introduced-for-2014/)
FIA sanctioned
European Formula 3 - http://www.fiaf3europe.com
GP3 Series - http://www.gp3series.com
GP2 Series - http://www.gp2series.com
Renault championships
Formula Renault 3.5, Eurocup Formula Renault 2.0 - http://www.worldseriesbyrenault.fr/en/Pages/Default.aspx
Formula Renault 2.0 Alps - http://www.renaultsportitalia.it/campionato/f-renault-2-0-alps
Formula Renault 2.0 NEC - http://necup.com
Formula Renault 1.6 NEC/Nordic - http://frcup.com , www.fia-nez.eu
National championships
Formula Renault UK - http://www.renaultsport.co.uk/championships/formularenaultbarc
Formula Ford UK - http://www.britishformulaford.co.uk
British Formula 4 - http://www.formula4.com
Formula ADAC (Germany) - http://www.adac-formel-masters.de
French Formula 4 - http://www.autosportacademy.com/gb/competition
Formula Abarth (Italy) - http://www.acisportitalia.it/ABARTH/
Euroformula Open (formerly Spanish Formula 3) - http://www.f3open.net
British Formula 3 - http://www.formula3.co
German Formula 3 - http://www.formel3.de/en/
Making things more confusing and adding to the level of redundancy, Renault plans to set up additional 1.6 litre series in the near future. (http://www.paddockscout.com/formula-renault-1-6-alps-series-introduced-for-2014/)