I use an external login box on my main site
www.ukbusinesslive.co.uk
its about half way down on the right looks like this
Not sure where i got the code from but its so easy to do, It won't come in a frame like my site as thats just customised, but it should be easy for you to add it to your site in a frame of some sort.
I added a picture, the option to sign in, and also a link to the register page, I find that a lot of people are clicking on the frame to take a look at the forums and then signing up, but its early days yet and i've not promoted the site as much.
I'll grab the code from my home page and stick it here so that you can get a rough idea of whats needed
Bear with me
here we are, Just change the red details for your own
Code:
<a href="http://www.ukbusinesslive.co.uk/forum"><img src="http://i475.photobucket.com/albums/rr114/ukbusinesslive/Forum-pic.jpg" border="0" ></a>
<a href="http://www.ukbusinesslive.co.uk/forum/register.php">Register Free NOW!</a>
<form action="http://www.ukbusinesslive.co.uk/forum/login.php?do=login" target="_new" method="post" onsubmit="md5hash(vb_login_password, vb_login_md5password, vb_login_md5password_utf, $show[nopasswordempty])">
<input type="hidden" name="do" value="login" />
<input type="hidden" name="url" value="/forum" />
<input type="hidden" name="vb_login_md5password" />
<input type="hidden" name="vb_login_md5password_utf" />
<input type="hidden" name="s" value="$session[sessionhash]" />
<label>Username:<input name="vb_login_username" type="text" id="login" tabindex="1" class="bginput" accesskey="u"/></label>
<label>Password:<input name="vb_login_password" type="password" id="password" tabindex="1" class="bginput" /></label>
<label>Remember me?<input type="checkbox" name="cookieuser" value="1" id="cb_cookieuser" tabindex="1" /></label>
<input name="submit" type="submit" id="submit" tabindex="1" value="Login" accesskey="s" class="button" />
</form>
If you want to try it before adding it into a frame just copy your edited code into notepad and save it to your desktop as forum.html or something similar, then you'll be able to test it by simply clicking on it
Hope that helps