The T2 bundle at CTAN:macros/latex/contrib/supported/t2
contains some other useful files, including support for Plain
TEX-based macro packages, support for BibTEX and MakeIndex (see
also the xindy program and package--highly recommended for
making indices with Cyrillic), support for the fontinst
package, mapping tables relating these Cyrillic font encodings (and
input encodings) to the Unicode character names and slots (these are
in the subdirectory enc-maps), and more!
To produce documented source listings of the T2 package, run
LATEX on the *.dtx and *.fdd files therein.
When typesetting Cyrillic texts, there is a tradition of using
Cyrillic letters (in some situations) within math formulæ, in
exactly the same way as most of the world uses Latin letters.
By default this does not work, because symbols declared with
\DeclareTextSymbol may not be used in math.
If you need within math to `transparently' typeset glyphs declared in font encoding definition files, then you could try using the experimental mathtext package, which is also in the T2 bundle. Note that this package uses up at least one additional math alphabet per font encoding. For this and other reasons, The LATEX3 Project Team considers that this experimental extension to LATEX's glyph-handling mechanisms should be used with caution; but please try it out and send us your opinions and ideas. Note that it is not included in the core of LATEX because both the coding and the interfaces are likely to change at some point in the future.
Finally, here are some pointers to further information:
http://www.cemi.rssi.ru/cyrtug
http://xtalk.price.ru/tex