I usually build against:
MySQL (duh)
PostgreSQL (if on the box)
XML support (expat)
OpenSSL
PDF support (pdflib)
Mcrypt support (libmcrypt)
Aspell (pspell)
DOMXML (libxml2/libxslt)
XSLT (Sablotron)
or, if you are lazy like me, here's my config script:
./configure \
--with-apxs2=/usr/local/apache2/bin/apxs \
--with-mysql=/usr/local/mysql \
--enable-bcmath \
--enable-calendar \
--enable-magic-quotes \
--with-pear \
--enable-track-vars \
--enable-versioning \
--with-zlib \
--enable-safe-mode \
--enable-trans-sid \
--with-gd \
--enable-gd-native-ttf \
--enable-exif \
--with-expat-dir=/usr/local \
--with-jpeg-dir=/usr/local \
--with-png-dir=/usr/local \
--with-freetype-dir=/usr/local \
--with-ttf=/usr/local \
--enable-sockets \
--with-openssl=/usr \
--with-pdflib=/usr/local \
--with-mcrypt=/usr/local \
--with-mssql=/usr/local \
--with-pspell=/usr/local \
--with-bz2 \
--with-zlib \
--with-zip \
--with-xml \
--with-dom=/usr/local \
--with-iconv=/usr/local \
--with-dom-xslt=/usr/local \
--with-dom-exslt=/usr/local \
--enable-xslt \
--with-xslt-sablot=/usr/local