There are quite a few TLDs offering 2-letter domains. These are the ones that can be registered without any restrictions:
.AG .CX .GD .GS .GL .HT .LC .MS .MU .PE .TC .TL .VG
.AG .CX .GD .GS .GL .HT .LC .MS .MU .PE .TC .TL .VG
I also heard about .gg and .je (Guernsey and Jersey - British Crown Dependencies off of the French coast) which can be registered directly at http://www.channelisles.net/applic/application.shtml (but they handle them manually so you have to wait a few days).