The captcha renderlet can be very easily used like this

<renderlet:CAPTCHA name="captcha" />

this would result in something like this :

But it can be fully customised with a code like this :

<renderlet:CAPTCHA name="captcha">
    <custom>class="texte"</custom>
    <label>Presentation Label</label>
    <copylabel>Copy the code</copylabel>
    <errormessage>Errrooooooooooooooorrrr</errormessage>
    <width>100</width>
    <height>30</height>
    <frame>false</frame>
    <bgcolor>0,255,0</bgcolor>
    <transparent_background>false</transparent_background>
    < font >
        <fontcolor>
            <random>true</random>
            <level>darker</level>
        </fontcolor>
        <family>Alanden_,luggerbu.ttf</family>
        <size>12</size>
    < font >
    <authchar>AZ</authchar>
    <crypteasy>
    <con>ZH</con>
    <vow>AU</vow>
    </crypteasy>
    <difuplow>false</difuplow>
    <nbchar>5</nbchar>
    <charspace>20</charspace>
    <charangle>10</charangle>
    <charup>true</charup>
    <noise>
    <pixel>200</pixel>
    <line>3</line>
    <noisecolor>charcolor</noisecolor>
    </noise>
    <imgtype>png</imgtype>
    <cryptsecure>md5</cryptsecure>
    <timer>
    <time>0</time>
    <action>image</action>
    </timer>
    <maxattempt>1000000000000</maxattempt>
</renderlet:CAPTCHA>

It would result in something looking like :

Here's the renderlet:CAPTCHA snippet