Well standard, as I'm sure you know, goes E-A-D-G-B-E (low to high). Drop D just changes the bottom E to a D. I'm sure you know this so not wanting to patronise, but i guess its god to cover it, right? Some examples of open tunings are: D-A-D-G-A-D or D-A-D-G-B-D and these give some pretty cool sounds. The tuning that Mastodon and Zakk Wylde often use is as follows B-A-D-G-B-E BUT this is often put down a whole step each time thus giving A-G-C-F-A-D. Tunings such as drop C are usually just the drop d tuning, then every string down a step or however many is required. There are many other open tunings, and well any tuning is possible! Hope this has been some use, and not just seen as patronising and a waste of time, if so not intentional, but hey I'm sure somone somewhere won't know this stuff even if you do dude.