This is a good question. Basically you have a few different categories:
GH Secretogues: ghrp-2, ghrp-6, and Ipamorelin are all popular growth hormone releasing peptides. When combined with a GRF (Growth hormone releasing factor) like mod grf 1-29 or cjc-1295 without DAC, then you get a great synergy and GH release from the pituitary. The basic desired dose is 1 mcg per kg of bodyweight. For me that's 100 mcg ea. up to 3 x day.
IGF-1 Drugs: igf1-lr3, igf-1 rh, des (1-3) igf-1, mgf, peg-mgf, igf2-lr3. Two schools of thought/methods of action for these drugs-
1) Insulin-like properties: The longer acting drugs are best for this: igf1-lr3 and peg-mgf. These produce results similar to humalog (used pre or post workout) but possibly with less of a tendency to desensitize the body to insulin.
2) Hyperplasia/site-enhancement: The shorter acting drugs are better for this. I just wrote all about this on my blog (see signature link). But, basically, a lot of the site-enhancement people saw early on was with igf-1 rh which is straight igf-1. DES (1-3) igf-1 is a new superior form of this. Again, check out my blog for more details!
Then there are the sarms- selective androgen receptor modulators. These work to build muscle in the same way as testosterone and other steroids but without the androgenic sides/less shut down of the hpta. Great for bridging imo.
This is just the very very basics. But, there is tons of info on this site and all over the web. This is my 2 cents only. No one knows it all! So, keep reading and keep learning!