I think we've all wanted to adjust / correct some of the digital photos that we've taken which were too dark or too bright. One way of sorting that out is using histograms in your photo application. Here are some tips on understanding and using histograms. Do check it out. I used histograms to correct Ramesh's b'day snaps which were a bit too dark. I cant say that I've cracked it but it did make 90% of the snaps look way better - you can now actually see faces!!
The Digital Photography Blog has several other wonderful tips on composition etc. Do take a look when you find the time.