What do you use to minimize and compress JavaScript libraries?

I use YUI Compressor. Seems to get the job done well!

I don't minimize JavaScript at all: gzip compression is good enough for me and has the additional benefit that error messages will still be useful.

I too use YUI Compressor. I have an ant task like this that I use in my projects:

<!--
YUI Compressor tasks 
http://www.julienlecomte.net/yuicompressor/README
-->
<property name="yuicompressor.jar"
           value="C:/devlibs/yuicompressor-2.2.4/build/yuicompressor-2.2.4.jar"/>

<target name="js.compress">
    <!-- Create min directory under js direcrtory if it doesnt exist -->
    <mkdir dir="${js-directory}/min" />

    <apply verbose="true" executable="java" parallel="false" failonerror="true">
        <fileset dir="${js-directory}" includes="*.js"/>
        <arg line="-jar"/>
        <arg path="${yuicompressor.jar}"/>
        <srcfile/>
        <arg line="-o"/>
        <mapper type="glob" from="*.js" to="${js-directory}/min/*-min.js"/>
        <targetfile/>
    </apply>
</target>

Dean Edward's packer achieves some pretty good compression ratios. It has command line implementations which allows it to be used in a continuous integration process.

I have tried YUI compressor before, but it gives me error message.

I suggest using JSMIN to minify your javascript:

http://www.crockford.com/javascript/jsmin.html

I use a simple (3-4 line) wrapper script around JavaScript::Minifier::XS.