Here are some of the best sites I've found for Collembola and other assorted mesofauna.
collembola.org The only site that dares to try and bring together the entirety of the world's info on Collembola with photos of thousands of species. It's a huge site and used by everyone.
List of UK and Irish Collembola. Extremely good overview of the UK and Ireland's Collembola, with distribution, descriptions and a taxonomic overview where possible of each species.
Jan van Duiren's springtail site. Great, unique photos, great info, in Dutch and English.
The FSC's site- Tom.bio. Tomorrow's biodiversity. The Field Studies Council is a fantastic organisation, promoting nature, mesofauna, soil health and their superb ID guides.
The Collembola page on Facebook. Keep up to date with posts linking to some of the latest research papers.
Massey University's Soil Bugs site. The only other site I've found so far, apart from this one, that gives an overview of the soil mesofauna. It also includes info on some macrofauna such as slugs, snails and harvestmen.
Sarah Lloyd's myxomycete site. A great site for all your slime mould/mold needs.
The Global Soil Biodiversity Initiative is a great scientific organisation, worth checking out.