I've been doing a little bit of research on protiens, and here's what I've come up with on the ones I'm most familiar with:
Animal proteins - These are the ones you have to be careful with. They can easily cause protien overload. Keratin protiens only strenghten the outer layer of your hair, with the exception of hydrolyzed protiens. These are broken down protiens that are small enough to penetrate the hair shaft and strengthen from within. When looking for protien conditioners, don't go for the ones that are "instant" or the ones that only require to leave the conditioner in your hair for 5 minutes or less won't really be as effective as a conditioner requiring to be left in for 10 -15 minutes or more.
Silk Amino Acid protiens - These are my favorite protiens. These penetrate the hair shaft in it's natural form, and some consider these protien bonds to be one of the stronger, if not, the strongest bonds. These protiens make your hair very soft as well.
Vegetable protiens - these protiens easily penetrate the hair shaft, mostly known to create shine and luster to your hair without residual buildup. Some even say that they help to maintain moisture.