It seems like every programming language designed feels the need to invent a new way of saying else if. Here are the ones I've encountered:

else if
C, C++ (but not cpp!), Pascal, csh (and tcsh and *csh), most sane languages. What else did you expect?

The problem with requiring this form is in languages which expect an explicit terminator for the "else" clause; after saying "if ... else if ... else if ... else if ... else ..., you need to close 4 "else" clauses, one on top of the other. Not only is it ugly, it means you need to indent each successive clause further in. This is not a Good Thing.

elif
cpp (more accurately, it's #elif there), sh (and zsh and bash and *[^c]sh...), Python
elsif
Perl.

EMERGENCY UPDATE!

dvdebug says Ada also uses this horrible form. I knew Perl stole the worst features of every language. But...
... Ada?
Uggh.
ifelse(a1, b1, t1, a2, b2, t2, ..., ak, bk, tk, f)
M4, of course. Expands to the first tj for which aj is equal to bj, or to f if none are equal.
EL SEIF, ELSEIF, ELS EIF, etc.
jclast points out that Fortran ignores whitespace. Just like you can say "GO TO" to mean "GOTO", you can say all the above (or even... "ELSE IF"...) to generate the keyword "ELSEIF".