I've noticed many players are hesitant to try alchemy, and for good reason. For those who aren't experts in this crafting skill, it can seem daunting to get into. Here are some resources I use that take a ton of the guesswork out of alchemy.
Skyrim Alchemy Lab
This site is an invaluable resource for those who want to know exactly how strong their concoctions will be. The menu on the right side lets you determine the exact magnitude of your concoctions based on your alchemy level, enchantments, and perks selected. Noticeably missing is 'Seeker of Shadows' which magnifies the effectiveness of thief skills, including alchemy level, by 10%.
Darliandor's Alchemy Lab
This resource is very useful for theory crafting potion and poison combinations. Every possible combination of poison and potion effect is available in the 4 separate effect drop down menus.
Simply pick your effects and see which ones match up. Once you've found the combination of effects you're satisfied with you can view below and see all the possible combinations of ingredients that will create your desired combination of effects. You can select different combinations of ingredients to see how/if they alter the effects in anyway. Very useful for alchemists looking to find the easiest combination of ingredients to gather.
Please, everyone, feel free to add to this. I'm sure there are many other online resources I missed that could prove just as useful.