Concording Greek and Latin texts

Concording a Latin text
A nice, free tool for quick concordances of one's own collection of Latin texts (or a single text) is Simple Concordance Program by Alan Reed. Easy to learn, easy to use. In SCP you can design your own alphabets, so you can also use it, e. g., for Greek texts in Betacode.

If you need a repository of Latin texts to concord, a bit more carefully proofread texts than those at the Latin Library can be found at the Itinera electronica, courtesy of the Universite Catholique de Louvain, Belgium. Itinera electronica have their own online concording facilities as well.

Another valuable tool (if you bring your own texts to it) is TAPoR, Text Analysis Portal for Research (a project based at McMaster University, and consisting of a network of six of the leading Humanities computing centres in Canada).

Concording a Greek text
A nice, free and cross-platform tool for concording Greek texts encoded in Unicode is Laurence Anthony's AntConc, available under http://www.antlab.sci.waseda.ac.jp/software.html.