The T2 bundle contains a variety of separate support functions, for using Cyrillic characters in LaTeX:

– the mathtext package, for using Cyrillic letters "transparently" in formulae
– the citehack package, for using Cyrillic (or indeed any non-ascii) characters in citation keys;
– support for Cyrillic in BibTeX;
– support for Cyrillic in Makeindex; and
– various items of font support.

The authors are Vladimir Volovich and Werner Lemberg.