Kenco is very good, as instant coffee goes, and I believe it's reasonably fair traded.
Cafe Direct is also excellent and is certainly environmentally and socially sound.
Both are available decaff (if that's what you want - I do in the evening) and Kenco comes in about four different varieties. None is quite like real coffee but (particularly if you drink it black with no sugar as I do) they are reasonable substitutes. Come to think of it, if you're going to add milk & sugar, does it matter much anyway? (ducks for cover!)