Yes, it is quite possible to tunnel between two networks. If the routers happen to be Linux machines, then even light-weight, unencrypted IPIP tunnel is possible. Or IPSec tunnel. Or any of the other VPN solutions.If you want it handled by the Routers, then you can get routers that support L2TP.
If you don't have Static IPs at each site, then that's a hurdle you'll need to overcome. I've never done it with Dynamic IPs, so I'm not sure what your options are there...
Could try this: https://secure.logmein.com/products/hamachi2/
Keep in mind, you can only go as fast as your slowest connection. If one link is 20/20 but the other link is only 5/2, your transfer speed will be 5/2.
Yeah, Hamachi is super easy and works well. It's also encrypted I believe, which is a bonus.
DD-WRT supports PPTP server/client and on the OpenVPN version it supports OpenVPN server/client.
It also supports DDNS, which will help with overcoming the dynamic IP's.
That will get you your layer 2 tunnel using networking hardware rather than your PC's running hamachi or something equivilent. This is advantagous because you could have devcies that are not capable of running hamachi talk to each other through the tunnel without running powering on your PC. eg. Xbox's PS3's etc talking to each other via the tunnel.