Pagina Annulorum I
Based in North Wales and environs,  this is a dedicated bunch of  re-enactors who recreate first century military and civilian life to a very high standard. They are a great group to work with, which is why the 14th is often to be seen in their company! Britain's most Northerly Roman re-enactment society, the Antonines have gone from strength to strength in the few short years since their foundation. They show immense dedication in everything they do - travelling hundreds of miles to attend events - and their Gask ridge march looks like becoming an important annual experience.

Originally a legionary group, these lads now specialise in the often neglected but actually vital auxiliary arm of the Roman military - specifically a milliary cohort of Vardulli from Hispania. They have a good  new website with some links that are well worth following.

Based at the impressively restored fort at Arbeia (South Shields) this auxiliary unit recreates military and civilian life on the Wall in the 3rd century AD - and it does so with great enthusiasm and remarkable accuracy. 3rd century kit makes an interesting change to standard 1st century fare whilst the group's civilian side is especially strong
This is the original Roman re-enactment society from which so much else has sprung. Experienced and professional.  Their website carries some informative articles on Roman military topics.
A first-rate group from the Netherlands often found working in Britain in association with other Roman re-enactment groups.. Their website is a definite must see for anyone interested in the Roman military or Rome in general - check out the comprehensive links section.
Lively group with interesting and informative website. Good to see the Imperium Romanum spreading so far South!

Excellent American legionary site with a wealth of useful and practical information, advice, links etc. Well worth a prolonged perusal.

Vexilatio Legionis XX Valeria Victrix and Cohors IX Batavorum

Legio XXX Ulpia Traiana Victrix
A Dutch Reenactment group including Exercitus Germanici Inferioris An Italian Historic research group based near Rome
  Batavian Auxilia (COH I BATAVORUM)
Based in Verona.
An experimental archaeology institute
specialising in III B.C. Gallic military reenactment. Includes a Celtic mercenary encampment.
A newly-formed (2011), but very experienced group of Roman/Germano-Celtic enthusiasts dedicated to the accurate reconstruction of the daily lives of Batavian auxiliary soldiers levied from the lower Rhineland and associated civilians attached to the Roman army during the mid-to-late First Century AD
Nemeton Ruis

Based in Milan.
Recreates III B.C. Gaulish culture. Crafts such as weaving, clay crafting and natural dyeing. Displays include a Celtic camp with apsects of the La Tène culture.

Aes Cranna

Based in Modena.III B.C. Reeanacts a Celtic settlement with fur working, tanning and metallurgy. Also involved in bronze smelting and working.