Yes, with VNC or with Apple's (commercial, $$) Apple Remote Desktop product. With VNC, you probably want to forward the connection over ssh, since it doesn't support encryption.
With OSX 10.4 it's built in. Under system preferences / Sharing, click "Remote Management" and in there there's a "Computer Management" button that has an option to turn on VNC. Definitely set a password. :) (10.3 and earlier you can find VNC servers)
You'll need a VNC client. Chicken of the VNC is what's usually recommended on OSX, but there's VNC clients for everything.
I believe the default port is 5900, so you could run "
ssh -L5900:localhost:5900 HOSTNAME" before starting up the VNC connection.
Actual X11 and
ssh -X will only help with X11 applications, which you probably don't care about on OSX.
Note: this isn't a second login, this is more taking over the screen remotely. There's ways with VNC on some platforms to do a second login instead of taking over the screen, but not sure how to do that on OSX.