IRAF

Some common and useful tasks (tasks inside packages in the format package.task) :

imarith: add, subtract, multiply or divide images by constants or other images
imexam: examine images in DS9 (keyboard commands: a, e, r, m, q etc.)
imcopy: copies images
imcombine: combines images
onedspec.splot: examine spectra in a graphics window (keyboard commands: a-a, w-a, k-k, d-d, q etc.)
onedspec.wspectext: turn 1D spectrum into a text file

Reduction of optical images (package: noao/imred/ccdred):

zerocombine: combine bias frames
flatcombine: combine flat frames
ccdproc: correct frames for bias and/or flat-field

Useful IRAF guides:
A beginners guide to using IRAF
http://users.utu.fi/kani/opetus/ht1/irafohje/index.html (In Finnish!)
https://users.utu.fi/kani/opetus/spektriohje/index.html (In Finnish!)