Some Canon cameras can be hacked with a tool called the Canon Hack Development Kit (http://chdk.wikia.com), which does interesting things like add manual controls and allow scripting. Do similar tools exist for other brands of cameras? HP cameras, in particular?
There is an ongoing project to analyse and de-encrypt the firmware of Pentax DSLRs:- Pentax Hack
There is an ongoing project to analyse and decrypt the firmware of a range of Nikon cameras: Official Nikon Hacks (d7000, D3100, D5100, V1, J1) and Nikon Tool development topic