I personally use GIMP. It's similar enough to photoshop without taking forever to load up (I'm cursed with slow computers) And it's free. http://www.gimp.org/
It doesn't always like my wacom, so when I start up, i do a 'test sheet' and if it doesn't register the pen or the pressure sensitivity, I go under the main menu, find preferences->input devices->reset input device settings (or something like that, I'm on a work computer now so I can't check). Close and restart the program and it should work fine. Happens maybe 1/4 the time I use the program, but it's a quick fix for a program that i like :)
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It doesn't always like my wacom, so when I start up, i do a 'test sheet' and if it doesn't register the pen or the pressure sensitivity, I go under the main menu, find preferences->input devices->reset input device settings (or something like that, I'm on a work computer now so I can't check). Close and restart the program and it should work fine. Happens maybe 1/4 the time I use the program, but it's a quick fix for a program that i like :)