Hi,
Attached is a function for SLIME which when run automatically balances all open sexps at point. It is similar to ILISP's CLOSE-ALL-LISP, which is bound to C-c ] or M-], I think, but smaller and easier to understand.
It is a little smarter than C-<Ret> in slime-repl, handles comments, reformats trailing right parens into one line, and DWIM when a region is active.
Strings are not yet handled because it can be quite costly, I think.
Take it if you like it :) (License is same as SLIME.)
Cheers, Michael