Are there are any VPN clients for Mac OS X that support L2TP?

Based on your question, I believe you're looking for a VPN client that runs L2TP over IPsec, the remote access solution promoted by Microsoft, starting with Windows 2000. Many VPN gateway vendors (including Check Point) initially implemented "vanilla" IPsec (tunnel mode) for host-to-network VPNs, but have since added L2TP over IPsec (transport mode) for interoperation with the embedded Windows VPN client. That's great for Windows users but, as you have found, can be a challenge for others.

Check Point sells a version of their VPN-1 Client for Mac OS 8.x/9.x, using vanilla IPsec. Apple's Mac OS X 10.3...

(Panther) now includes an embedded VPN client that supports L2TP over IPsec. You're stuck in between with Mac OS 10.2.8 (Jaguar). Jaguar incorporated embedded VPN code but not a graphical user interface to IPsec. Panther expands the Mac's embedded VPN client, adding a graphical user interface to L2TP/IPsec, and is really your best option.

If you can't upgrade to Panther, you may want to try a third-party L2TP VPN Client for Jaguar. Check Point-compatible VPN clients for Mac OS 10.2 listed at Apple's Web site include Lobotomo IPsecuritas and Equinux VPN Tracker, although I can't tell from a quick look at product documentation whether these clients support L2TP/IPsec or just IPsec. You can also try your hand at configuring Jaguar's embedded IPsec code using the command line interface. To learn more about configuring Jaguar's IPsec, see this PPT presentation by Paul Hoffman of the VPN Consortium. Keep in mind that you'd still need to configure L2TP on top of IPsec to pair with your VPN gateway's policy.