I don't really know where to start with post-processing my images. I normally just straighten, crop and sharpen. Do people know of any good resources on the internet for getting a grip with Photoshop and similar tools?
I recommend the Photoshop manual, and then lots and lots of practice/experimentation.
I know this is not the answer you really want (and it's unlikely to get the bounty ;) but it's my opinion (and I stress this is just an opinion) that tutorials will only teach you how to use certain steps to produce a certain result.
There is no substitute for learning everything the long way, finding out for yourself why to make a change rather than simply how. Training your eyes to analyse an image, learning when to trust your eyes and when to go by the numbers. Flexing your creativity. An experienced Photoshop artist will be able to look at any tutorial end image, take the source images and recreate it, probably using different steps to get the same result. There are so many tutorials that promise instant results and I can see the appeal, but this ability to me is far more valuable.
I recommend http://www.Lynda.com - great videos about Photoshop, Lightroom, Bridge and hundreds of other design/photo software for just $25/month.
I recently found Digital Photography School to have a good selection of tutorial on post processing as well as other areas of photography.
I've used Ron Bigelow's photoshop tutorials quite a bit. He covers just about everything I care about and then some... He also has a bunch of other articles/tutorials on other photographic subjects that are interesting.
For Photoshop, check out Scott Kelby's Photoshop User TV, they have weekly videos and other resources available there too: http://kelbytv.com/photoshopusertv/
I also recommend going to forums that have post-processing contests, where everyone post-processes one image and explains their technique, and the submitter chooses a winner.
photocamel, photoforum, etc.
One overlooked skill is applying the best post-process for a specific image, and this helps immensely.
The tutorials at Chromasia are really good. You have to pay for a yearly subscription to get most of them, but I think it's worth it. They're extremely thorough with a lot of examples.
Free sample tutorial: http://www.chromasia.com/tutorials/online/curves/
I recommend photo walkthrough tutorial. They have a very good series of video podcast on post processing using Adobe Photoshop and Lightroom.
Definitely get a subscription to www.lynda.com. Awesome tutorials on software AND classes AND inspirationals from professionals.
Matt Kloskowski posts free Lightroom tutorials at Lightroom Killer Tips. I learned a lot from his material.
Some post-processing tutorials are at http://photo.tutsplus.com/category/tutorials/post-processing/
I learner some killer tips from these free tutorials at www.photoshopsupport.com/tutorials.html
The website is literally packed with tips and tricks explained by professionals and great photographers. It is also linked to other websites with tutorials.
If you consider a more in-depth Photoshop exploration, consider buying some literature as well. You will find plenty of Photoshop books and guides on Amazon.
Like many people have already mentioned, Lynda.com is what I recommend. Initially, I've learnt post processing from YouTube videos because of the fact that it was free. But the problem was I never knew where to start and what video to watch next. I watched videos randomly and, somehow, learnt how to edit pictures well. However, after some time I just couldn't improve my skills. There were several videos on YouTube but most were what everyone else has already done. So then, one day I tried Lynda. I should tell you, I'm damn good now, and I give entire credit to Lynda tutorials.
There are structured courses. So, I was able to go through without the struggle of deciding what to watch next. To be honest, what I wanted to learn was "when to use what tool, how to use it and why should I use it". Lynda offered exactly that!
If you ask me where to start, I recommend courses by Chris Orwig on Lynda.com, as I felt his videos are simple and easy to grasp. If you want more, take courses by Ben Long.
You can sign up for Lynda free trial and check the courses.